feat: introduce Forge 2.0 durable workflow control plane

[eshulman2]

Forge 2.0 durable workflow control plane

This is a single-commit consolidation of the validated PR stack and integration hardening.

It introduces:

• versioned observations, commands, effects, and station contracts;
• reconciliation of webhook and poller ingress;
• durable idempotent Jira/source-control/repository effects with recovery;
• governed, pinned declarative workflows;
• execution/timeline and effect operator APIs;
• strict structured agent-output validation;
• shared implementation execution and fixes validated through Feature, Task Takeover, and Bug workflows.

It declares package version 2.0.0.

Supersedes the stacked PRs #324, #325, #326, #327, #328, #329, #330, #331, and #332. Keep those PRs open until this replacement has been reviewed and approved.

Validation

• uv run pytest tests/unit/workflow/test_pr_state.py -q — 21 passed
• uv run ruff check src/forge/workflow/pr_state.py tests/unit/workflow/test_pr_state.py
• Feature, Task Takeover, and Bug workflows were exercised against the integration branch before consolidation.

Documentation

• docs/forge-2.0-control-plane-guide.md
• docs/architecture/forge-2.0-control-plane-design.md

Compatibility

This is a deliberate major-version cutover. Legacy direct execution paths are removed; drain, resolve, or explicitly migrate old active workflows rather than assuming resumption.