[codex] Add trace summarizer registry

[roackb2]

Summary

Adds the M8 observability foundation for trace summarization:

• moves chat trace summary behavior into src/core/observability/trace-summarizers.ts
• keeps src/core/chat/trace-summary.ts as a compatibility wrapper for existing imports
• adds semantic trace convention constants under src/core/observability/semantic-conventions.ts
• exports the observability registry from the public package API
• adds behavior-locking tests for existing summary output, skipped event behavior, registry overrides, and compatibility-path behavior

Why

This replaces the central trace summary switch with a typed registry so future domains can add or override trace summarization through composition instead of editing one growing switchboard. Current turn summary strings are intentionally preserved.

Validation

• yarn test:unit
• yarn vitest run src/__tests__/integration/chat/chat-runtime.test.ts
• yarn eslint
• yarn typecheck
• yarn build

Note: the package script yarn test:integration src/__tests__/integration/chat/chat-runtime.test.ts runs the whole integration suite because of the script shape. In the sandbox, unrelated suites fail on temp git identity and localhost EPERM; the direct targeted Vitest command above passes.