feat(coding-agent): add built-in message renderer transforms

[quick-ricon]

Summary

• add a typed registerBuiltInMessageRenderer("user" | "assistant", transform) extension API
• compose renderer transforms once in extension load order, with later transforms wrapping earlier ones
• pass complete role-specific messages and live render options while preserving Pi's transcript lifecycle roots
• support native (default) and extension-owned (self) card shells with guarded fallback
• isolate extension-facing message snapshots so renderer mutations cannot alter session or model context
• document the API and include a compiled example

This is a display-only extension point. It does not alter persisted messages or model context, and it deliberately excludes viewport/follow-mode behavior.

Validation

• initial focused renderer, user-message, assistant-message, and extension-runner tests: 60/60 passed
• final-head renderer, user-message, and assistant-message tests: 22/22 passed, including the nested-mutation regression from fix: isolate built-in renderer messages #41
• focused TypeScript check: passed
• scoped Biome check: passed
• git diff --check: passed
• signed commit and push preflights: passed

Local broad-gate degradation

npm run check reaches root TypeScript and then fails on two unrelated model IDs already broken on a clean origin/main worktree:

• packages/ai/test/context-overflow.test.ts:130 references removed github-copilot/gemini-2.5-pro
• packages/ai/test/openai-completions-tool-choice.test.ts:199 references removed groq/qwen/qwen3-32b

./test.sh also could not provide a clean local broad-suite result because isolated child fd/rg calls hit the local untrusted-mise boundary and spawned CLI tests expect missing dist artifacts. The repository contract forbids generating those artifacts with a build. No renderer-focused test failed.

Hosted CI run 31017590014 confirmed the same two clean-main TypeScript failures and therefore stopped before its test phase.

No changelog entry is included because CONTRIBUTING.md says not to edit the changelog in pull requests.

[junior-ricon]

Change request

packages/coding-agent/src/modes/interactive/components/built-in-message-renderer.ts:56 passes the live AgentMessage object directly into extension transforms. Those are the same objects retained by the agent/session path, and assistant streaming copies are shallow, so a transform that assigns nested content mutates persisted transcript/model context despite this API's display-only contract. I reproduced this by changing UserMessage.content[0].text inside a transform; the source message changed. Please merge or adapt #41, which snapshots messages before entering the transform chain and adds the nested-mutation regression.

Adversarial coverage

I read the exact diff before the PR body, then checked existing discussion and CI. I stress-tested role typing, extension registration/load and composition order, stable composition state, default/self ownership, invalid/throwing transforms, streaming/final/restored and expansion updates, resize behavior, outer OSC lifecycle roots, exports, docs, and test evidence.

The focused renderer, user-message, and assistant-message suites pass (22/22, including the fix-it regression). The extension-runner suite remains locally uncollectable because the Git archive lacks generated packages/ai/src/providers/data/amazon-bedrock.json. Hosted CI failed on the disclosed clean-main model-data TypeScript errors before tests; I found no additional PR-head-specific CI failure.