chore(cargo): bump version to 0.8.0

[rocketman-code]

Summary

• Bump piano and piano-runtime to 0.8.0
• Update CHANGELOG.md with all changes since 0.7.0
• Regenerate Cargo.lock

What's in 0.8.0

Milestone 3 (UX) complete. Every user-facing touchpoint audited against UX principles.

New features: --exact flag, --list-skipped flag, piano tag list mode, piano diff no-args, auto-detect project root, structural color, recovery guidance in errors

Fixes: diff sort order, duplicate errors, path leaks, cascading error suppression, qualified name matching, tag error messages

Changed: column order (Self leads), Total removed from default view

Full changelog in CHANGELOG.md.

After merge

Tag v0.8.0 on the merge commit, then publish piano-runtime first, then piano.

[rocketman-code]

Version bump commits should contain only the version bump itself (Cargo.toml version fields and lockfile). Any other changes (CHANGELOG, docs, etc.) should be in separate commits.

Please split this PR so the version bump commit contains only:

• Version bump in Cargo.toml
• Version bump in piano-runtime/Cargo.toml
• Regenerated Cargo.lock

Other changes (CHANGELOG updates, etc.) should be separate commits per the release checklist in docs/standards/releases.md.