feat: add amplifier-agent-adapter-designer bundle

[manojp99]

What this bundle does

Provides /mode amplifier-agent-adapter-designer — a runtime mode that turns any Amplifier session into a focused workspace for designing adapters between host applications and amplifier-agent.

Why it lives in this repo

The bundle is tightly coupled to the engine: it knows the three integration surfaces (Python SDK, TypeScript SDK, chat-completions HTTP server), the three existing host adapter case studies (paperclip, nanoclaw, opencode), and the cross-cutting concerns every integrator hits (credentials, MCP config, bundle cache priming, protocol version pinning, env allowlist, workspace isolation). Shipping it inside amplifier-agent keeps the knowledge co-versioned with the engine that generates it.

Mechanism mix

• Mode (user-facing entry point): /mode amplifier-agent-adapter-designer
• Expert agent (adapter-design-expert) — context sink carrying the ~2,500-token integration reference
• Two context files: thin awareness pointer (always-on) + full reference (agent-only)
• Behavior YAML wires it all together

No recipe, no skill. Adapter design is a conversational, branching exercise — a recipe would impose rigid step-gating on something that needs to breathe.

Context discipline

Layer	When loaded	Approximate weight
adapter-design-awareness.md	always-on (thin pointer)	~200 tokens
Mode body	only while mode active	~900 tokens
integration-reference.md	only inside expert agent's child session	~2,500 tokens

The full reference never touches the root session.

Verification

Behavioral model in docs/BEHAVIORAL_MODEL.md covers 10 scenarios — fresh activation, surface selection, case-study deep dive, cross-cutting concerns walkthrough, error handling, mode-not-active isolation, protocol skew. All pass.

Open items to resolve before ready-for-review

1. Verify the @foundation:context/shared/common-system-base.md path matches the deployed foundation fork.
2. Confirm lazy mode discovery works for composed bundles; if not, add hooks-mode search_paths config to behaviors/agent-adapter-designer.yaml.
3. amplifier-agent-py install path currently says "not yet on PyPI, install from git source" — will need update when PyPI publishes.
4. Verify protocol version 0.3.0 in integration-reference.md matches current SDK source.
5. Verify displayMode (camelCase) matches actual amplifier-agent-ts API surface vs Python's display_mode (snake_case).
6. Confirm bundle namespace agent-adapter-designer isn't already taken in the registry.

Draft status

These 6 open items should be resolved and the bundle exercised via test-drive before ready-for-review.