chore(release): bump version to 0.6.1

[lostmartian]

What

Bumps all three distributables to 0.6.1 together and cuts the changelog section for #61 (Claude Code plugin) and #62 (npx agentdiff, plus the uv/pipx/Python fallbacks).

File	Version
src/agentdiff/__init__.py (PyPI agent-trajectory-diff)	0.6.0 → 0.6.1
npm/package.json (npm agentdiff)	0.6.0 → 0.6.1
plugins/agentdiff/.claude-plugin/plugin.json	0.6.0 → 0.6.1

Why

• npm: npx agentdiff needs the package on npm, and the launcher pins the PyPI release of the same version.
• Plugin: Claude Code pins plugin users to plugin.json's version, so this bump is what delivers the new pipx/Python fallbacks to people who already installed the plugin.

Roadmap: P1.2 and P1.7.

How

After merge, pushing the v0.6.1 tag runs publish.yml:

1. build;
2. PyPI (trusted publishing);
3. npm, after checking the tag matches npm/package.json and running the launcher tests. Published with --provenance using NPM_TOKEN, which is now set. This first publish claims the agentdiff name;
4. GitHub release.

Testing

make lint                  -> All checks passed
uv run python -m pytest    -> 461 passed (includes the three-way version-sync test)
cd npm && npm test         -> 4 passed
uv build                   -> agent_trajectory_diff-0.6.1 wheel + sdist
npm pack --dry-run         -> agentdiff@0.6.1, 3 files
uvx --from ./dist/...0.6.1...whl agentdiff --since 7d -> report renders
claude plugin validate --strict .  -> ✔ Validation passed

Checklist

• Exactly one logical change in this PR
• make lint passes
• uv run python -m pytest is green
• Docs updated if user-facing
• CHANGELOG.md entry added under [Unreleased] (if feat/fix)
• Conventional title: type(scope): summary