kit v0.6.0

Ship audit release. Root cause fix for checksum verification failures
on own tools, plus 8 additional findings from comprehensive code audit.

kit check was downloading cosign bundles (.bundle files) instead of
binaries for own tools. The release link matching used contains()
which matched kit-darwin-arm64.bundle before kit-darwin-arm64 because
the bundle link appeared earlier in the release links array.

Fixed: exact name match first, then URL ends_with(/asset_name) fallback.

- jq advisory filter: escape dots in version before regex interpolation
- Bundle URL construction: append .bundle instead of replace() which
  could corrupt URL path
- resolve_installed_sha: return None when binary not found (don't
  store registry checksums in the binary_sha256 field)
- cmd_upgrade: remove stale [tool.checksums] after version bump
- check_crates: exact name match instead of prefix match
- URL validation: reject embedded newlines
- cmd_pin: validate version before saving
- apply commit: include flagged updates in commit message

125 tests. 0 clippy warnings.
6 adversarial reviews + 1 ship audit, 55 total findings addressed.