[codex] Stabilize reactions build and install docs

[nanami-he]

Summary

• Make dist/reactions-pool.json deterministic by collecting source reaction pools directly, deduping, and sorting before writing the artifact.
• Add tests covering sorted reaction output and source-pool collection without random sampling.
• Replace the README quick start with a clearer Install section covering npx, Claude Code plugin install, and source install paths.
• Mark the completed G0 PRD items and add a 2026-05-01 progress note.

Validation

• bun test -> 319 pass, 0 fail
• bun run build twice -> dist/reactions-pool.json SHA-256 stayed 4fc02c0d6a9cdd9a14fca0dacf0260d29b18f38c2293259dbdc4a4f867361d51
• NPM_CONFIG_CACHE=/private/tmp/petsonality-npm-cache npm pack --dry-run -> package contains expected 15 files, including dist/reactions-pool.json and dist/server.js

Notes

Windows smoke testing remains intentionally out of scope for this PR.