chore(SDK-971): improve tsdoc tooling after first backfill round

[mariechatfield]

Summary

Jira: SDK-971

Improvements to the internal TSDoc tooling after running the first phase of the backfill. Adjustments were driven by what we learned from actually running the agents at scale.

• tsdoc-api-documenter agent: Expanded instructions with more precise rules for when to use @public vs @internal, how to handle re-exports, and common pitfalls (e.g. don't document symbols only exported for testing). Removed the tsdoc-file skill dependency — the agent now has the full guidance inline.
• tsdoc-backfill agent: Tightened scope rules so the agent doesn't drift into undocumented territory or over-document internal symbols.
• tsdoc-directory skill: Simplified — removed redundant guidance now covered by the agents directly.
• tsdoc-file skill: Deleted — was superseded by the improved agent instructions.
• knip command + script: Added .claude/commands/knip.md and .claude/scripts/knip-fix.sh to support running knip to surface unused exports and files after a backfill round, as a quality check.
• knip config: Added .reports/config/knip.json with project-appropriate ignore patterns (test files, Storybook stories, etc.).
• tsdoc stubs (build/tsdoc-stub.ts, build/tsdoc-stub-lib.ts): Simplified — removed unused options and dead paths exposed by the backfill work.
• eslint config: Minor update to allowlist path for TSDoc coverage tracking.

Test plan

• Run npm run build to confirm the stubs still compile cleanly
• Spot-check that the knip script runs: npx knip --config .reports/config/knip.json

[mariechatfield]

Support to run /knip <directory> directly, so that you can do a targeted removal of any unused code. Uses npx knip so we don't have to increase our developer dependencies. The full run is... a lot of unused code removal. Feels better to do in groups as we document