docs: align CLAUDE.md, CONTRIBUTING.md, and CHANGELOG with pnpm migration (#233)

* docs: update CLAUDE.md and CHANGELOG for pnpm migration

Refs #232

- Replace npm with pnpm in CLAUDE.md dev/release workflow snippets
  (install, build, test, link, lint, format, release).
- Add a Security entry to CHANGELOG noting the 24-hour package
  quarantine (`minimumReleaseAge: 1440`) introduced with the pnpm
  migration to reduce supply-chain attack risk.

* docs: shorten changelog entry for pnpm migration

* docs: update CONTRIBUTING.md for pnpm and trim duplicated architecture

Refs #232

- Replace npm with pnpm across dev setup, testing, and release sections
- Add a short note on the 24h minimumReleaseAge supply-chain quarantine
  so contributors aren't surprised when a fresh dependency bump waits
- Fix stale references: keytar → @napi-rs/keyring, and the outdated
  `mcpc <url> session @name` CLI syntax in the session-lifecycle example
- Trim the architecture overview section: the protocol/IPC/lifecycle
  detail duplicated CLAUDE.md and was already rotting. Replaced with a
  high-level module map plus a pointer to CLAUDE.md for the deep dive.

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Co-authored-by: Claude <noreply@anthropic.com>