docs: document 5.0.1 changes and refresh stale references

[pjdoland]

Summary

The version was bumped to 5.0.1 (commit 7d512cc) and three PRs merged since the 5.0.0 release, but the CHANGELOG still ended at [5.0.0] with an empty [Unreleased], and one CONTRIBUTING reference was left pointing at the removed fastmcp dependency. This adds the missing [5.0.1] CHANGELOG section and fixes the stale reference so the docs reflect the current state of the code.

Solution

CHANGELOG.md: new [5.0.1] - 2026-05-24 section between [Unreleased] and [5.0.0], plus footer compare-links ([unreleased] repointed to v5.0.1...HEAD, new [5.0.1] line). Entries:

• Changed: GitHub Copilot Codex chat models route through the /responses endpoint (feat(copilot): route Codex chat models through /responses endpoint #341); npm package scope renamed to @plmbr/notebook-intelligence (feat: rename npm package prefix to @plmbr #342).
• Fixed: AI-generated chat links open in a new tab instead of replacing the Lab UI (fix(chat): open AI-generated links in new tab instead of replacing Lab UI #347).

CONTRIBUTING.md: mcp_manager.py is now described as using the official mcp SDK rather than fastmcp (the swap shipped in 5.0.0 via #324; the module imports mcp, confirmed).

Deliberately left unchanged

• README.md "5.0.0 migration note" link and docs/admin-guide.md "As of 5.0.0 ... mcp SDK" line: both reference 5.0.0 as correct historical fact. 5.0.1 introduces no traitlet, env-var, REST route, or on-disk-format change and no migration steps, so pointing at the 5.0.0 note remains accurate.
• The #343 blog-URL refresh, the version-bump commit, and a lockfile lint fix are chores/docs with no user-facing surface, so they get no CHANGELOG line.

Testing

• prettier --check CHANGELOG.md CONTRIBUTING.md: both pass.
• Every factual claim in the new entries was verified against the code: gpt-5.3-codex is the real model id (12 occurrences in the test suite); _RESPONSES_TERMINAL_ERROR_EVENTS contains exactly the four events listed; mcp_manager.py imports mcp; package.json name is @plmbr/notebook-intelligence while the pip name is unchanged.
• Two rounds of multi-agent review (changelog accuracy, completeness, prose/house-style, cross-doc consistency, link integrity, tone). Round 1 corrected the model id (gpt-5-codex → gpt-5.3-codex), completed the error-event list, and removed an em dash from the touched CONTRIBUTING line; round 2 came back clean.

Risks / follow-ups

• Docs-only change; no code or behavior affected.
• The CONTRIBUTING bullet now uses a colon separator while its sibling bullets use em dashes; this is a deliberate house-style choice on the touched line, not an error.