docs: surface diagnostic traps and prepare v0.2.0

[tmusser]

Why

The diagnostic implementation is strong; this follow-up improves discoverability and turns the current package version into a formal GitHub release.

What changed

• Expand the README statistical-diagnostics section with direct links to all four runnable traps and their expected REVIEW/BLOCK outcomes.
• Consolidate the split changelog notes into a single v0.2.0 entry dated 2026-07-12.
• Add curated v0.2.0 release notes.
• Add a one-time merge-triggered workflow that creates tag v0.2.0 at the merged commit and publishes a formal GitHub Release.

Release behavior

Merging this PR into main changes .release/v0.2.0.md, which triggers the release workflow. The workflow:

1. Checks whether v0.2.0 already exists.
2. Exits safely if it does.
3. Otherwise creates the tag at the merge commit and publishes chart-contract v0.2.0 using the curated notes.

The repository declares package version 0.2.0, and no v0.2.0 tag was resolvable before this PR.

Verification

• Full pytest suite: passed
• Legacy CLI REVIEW/BLOCK/warnings-as-errors checks: passed
• Statistical diagnostic trap checks: passed
• PR is mergeable
• The release workflow is restricted to pushes to main that change .release/v0.2.0.md; it cannot publish from the PR branch