feat(ensemble): UI workflow + OpenRouter backend + explicit chapter selection

.specify/extensions.yml

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+installed:
+- agent-context
+settings:
+  auto_execute_hooks: true
+hooks:
+  after_specify:
+  - extension: agent-context
+    command: speckit.agent-context.update
+    enabled: true
+    optional: true
+    priority: 10
+    prompt: Execute speckit.agent-context.update?
+    description: Refresh agent context after specification
+    condition: null
+  after_plan:
+  - extension: agent-context
+    command: speckit.agent-context.update
+    enabled: true
+    optional: true
+    priority: 10
+    prompt: Execute speckit.agent-context.update?
+    description: Refresh agent context after planning
+    condition: null

.specify/extensions/.registry

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+{
+  "schema_version": "1.0",
+  "extensions": {
+    "agent-context": {
+      "version": "1.0.0",
+      "source": "local",
+      "manifest_hash": "sha256:9a1dc02d2d0139bb03860392ecacef79183be2c442feda2f9ccaa4e5907b1e47",
+      "enabled": true,
+      "priority": 10,
+      "registered_commands": {
+        "claude": [
+          "speckit.agent-context.update"
+        ]
+      },
+      "registered_skills": [],
+      "installed_at": "2026-06-27T21:48:08.109321+00:00"
+    }
+  }
+}

.specify/extensions/agent-context/README.md

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+# Coding Agent Context Extension
+
+This bundled extension manages the **coding agent context/instruction file** (e.g. `CLAUDE.md`, `.github/copilot-instructions.md`, `AGENTS.md`, `GEMINI.md`, …) for the active integration.
+
+It owns the lifecycle of the managed section delimited by the configurable start/end markers (defaults: `<!-- SPECKIT START -->` / `<!-- SPECKIT END -->`).
+
+## Why an extension?
+
+Not every Spec Kit user wants Spec Kit to write into the coding agent's context file. Extracting this behavior into a dedicated extension lets users:
+
+- **Opt out** entirely with `specify extension disable agent-context` — Spec Kit will then never create or modify the agent context file.
+- **Customize the markers** by editing `.specify/extensions/agent-context/agent-context-config.yml` — both the Python layer and the bundled scripts honor the same `context_markers` value.
+- **Synchronize multiple agent anchors** by setting `context_files` when a project intentionally uses more than one coding agent context file, such as `AGENTS.md` and `CLAUDE.md`.
+- **Refresh on demand** with `/speckit.agent-context.update`, or automatically through the hooks declared in `extension.yml` (`after_specify`, `after_plan`).
+
+## Commands
+
+| Command | Description |
+|---------|-------------|
+| `speckit.agent-context.update` | Refresh the managed section in the agent context file with the current plan path. |
+
+## Configuration
+
+All configuration flows through the extension's own config file at
+`.specify/extensions/agent-context/agent-context-config.yml`:
+
+```yaml
+# Path to the coding agent context file managed by this extension
+context_file: CLAUDE.md
+
+# Optional list of coding agent context files to manage together.
+# When non-empty, this takes precedence over context_file.
+context_files:
+  - AGENTS.md
+  - CLAUDE.md
+
+# Delimiters for the managed Spec Kit section
+context_markers:
+  start: "<!-- SPECKIT START -->"
+  end: "<!-- SPECKIT END -->"
+```
+
+- `context_file` — the project-relative path to the coding agent context file, written by `specify init` and `specify integration install`.
+- `context_files` — optional project-relative paths to multiple coding agent context files. When non-empty, the list takes precedence over `context_file`. Absolute paths, backslash separators, and `..` path segments are rejected.
+- `context_markers.start` / `.end` — the delimiters around the managed section. Edit these to use custom markers.
+
+## Requirements
+
+The bundled update scripts require **Python 3** with **PyYAML** for YAML/upsert processing (PowerShell can also use `ConvertFrom-Yaml` when available).
+
+PyYAML ships with the `specify` CLI and is normally available via the same `python3` interpreter. If a hook reports *"PyYAML is required … not available in the current Python environment"*, it means the system `python3` differs from the one used to install Spec Kit. To resolve, run:
+
+```bash
+pip install pyyaml
+# or target the specific interpreter Spec Kit uses:
+/path/to/speckit-python -m pip install pyyaml
+```
+
+## Disable
+
+```bash
+specify extension disable agent-context
+```
+
+When disabled, Spec Kit skips context file creation, updates, and removal (the gates are inside `upsert_context_section()` and `remove_context_section()`).
+Disabled projects also ignore stale `context_files` values during command rendering so disabling the extension remains a complete opt-out.

.specify/extensions/agent-context/agent-context-config.yml

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+context_file: CLAUDE.md
+context_files: []
+context_markers:
+  start: <!-- SPECKIT START -->
+  end: <!-- SPECKIT END -->

.specify/extensions/agent-context/commands/speckit.agent-context.update.md

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+---
+description: "Refresh the managed Spec Kit section in coding agent context file(s)"
+---
+
+# Update Coding Agent Context
+
+Refresh the managed Spec Kit section inside the active coding agent's context/instruction file (e.g. `CLAUDE.md`, `.github/copilot-instructions.md`, `AGENTS.md`).
+
+## Behavior
+
+The script reads the agent-context extension config at
+`.specify/extensions/agent-context/agent-context-config.yml` to discover:
+
+- `context_file` — the path of the coding agent context file to manage.
+- `context_files` — optional project-relative paths for multiple coding agent context files. When non-empty, the script updates each listed file and the list takes precedence over `context_file`.
+- `context_markers.start` / `.end` — the delimiters surrounding the managed section. Defaults to `<!-- SPECKIT START -->` and `<!-- SPECKIT END -->` when the field is missing.
+
+It then creates, replaces, or appends the managed block so that the section points at the most recent plan path when one can be discovered (`specs/<feature>/plan.md`).
+
+If `context_files` and `context_file` are empty, the command reports nothing to do and exits successfully. Context file paths must stay project-relative; absolute paths, Windows drive paths, backslash separators, and `..` path segments are rejected.
+
+## Execution
+
+- **Bash**: `.specify/extensions/agent-context/scripts/bash/update-agent-context.sh [plan_path]`
+- **PowerShell**: `.specify/extensions/agent-context/scripts/powershell/update-agent-context.ps1 [plan_path]`
+
+When `plan_path` is omitted, the script auto-detects the most recently modified `specs/*/plan.md`.

.specify/extensions/agent-context/extension.yml

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+schema_version: "1.0"
+
+extension:
+  id: agent-context
+  name: "Coding Agent Context"
+  version: "1.0.0"
+  description: "Manages coding agent context/instruction files (e.g., CLAUDE.md, copilot-instructions.md) with project-specific plan references and configurable markers"
+  author: spec-kit-core
+  repository: https://github.com/github/spec-kit
+  license: MIT
+
+requires:
+  speckit_version: ">=0.2.0"
+
+provides:
+  commands:
+    - name: speckit.agent-context.update
+      file: commands/speckit.agent-context.update.md
+      description: "Refresh the managed Spec Kit section in the coding agent context file"
+
+hooks:
+  after_specify:
+    command: speckit.agent-context.update
+    optional: true
+    description: "Refresh agent context after specification"
+  after_plan:
+    command: speckit.agent-context.update
+    optional: true
+    description: "Refresh agent context after planning"
+
+tags:
+  - "agent"
+  - "context"
+  - "core"