Link manually published GitHub issues to local suggestions

[Widthdom]

Adds cdidx suggestions link <id> --repo <owner/name> --issue <number-or-url> so findings published manually can be associated with their existing GitHub issue. Several suggestions can share one issue without creating remote issues or inventing submission attempts, synchronization, or implementation completion.

The association is validated and saved atomically with manual provenance and a UTC audit timestamp. Writes are checked against the 8 MiB reader limit before replacing the previous store. Same-identity repeats preserve history; conflicting reassociation and in-flight submissions are rejected. Full list/show/export output includes the association, issue-draft duplicate guidance identifies existing publication, and later duplicate submissions return the existing issue without invoking issue creation.

Linking works offline and intentionally performs no remote verification. The documentation explains that existence, issue-versus-PR identity, and upstream state must be checked separately.

Validation

• Release solution build: zero warnings/errors.
• Focused net8 tests: 790 core CLI/store/help tests and 204 GitHub/MCP/subprocess suggestion tests passed.
• Focused net9 tests: 951 passed; 42 existing production-runtime-only subprocess tests skipped as designed.
• After integrating latest main, rebuilt successfully and reran 145 focused suggestion/GitHub/MCP integration tests on each of net8/net9; all passed.
• Final store/CLI/schema suite after the capacity-boundary guard: 143 passed on each of net8/net9.
• Codex adversarial review completed in two rounds; final result: No blocking/actionable issues found.
• Root and workspace index checks both passed after the final commit, with complete reference graphs.
• Changed C# formatting check and changelog fragment validator passed.
• Repository-built CLI smoke: link to existing Link manually published GitHub issues to local suggestion records #5350, repeat without timestamp changes, and export existing-publication evidence with zero submission attempts. A near-capacity store rejects the link with E021_SUGGESTION_STORE_UNAVAILABLE while preserving readable history without quarantine.

Documentation

• Added bilingual docs/suggestion-issue-linking.md; updated the corresponding English/Japanese sections in USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Changelog fragment: changelog.d/unreleased/5350.added.md.

Fixes #5350