Update CI workflow to copy notes from fork after merge

[Ngineer101]

Fixes: #411

Summary

• Adds fork-aware notes handling in CI merge flows so AI authorship from forked PRs/MRs is preserved.
• Detects fork source repos in both GitHub and GitLab CI contexts, fetches refs/notes/ai from the fork, and merges/copies notes locally before attribution processing.
• Extends git-ai ci local merge with --fork-clone-url and adds integration coverage in tests/ci_fork_notes.rs for fork-notes and no-notes scenarios.

Why

Fork merges (especially merge-commit paths) can miss authorship notes if only the base remote is checked; this ensures attribution survives common fork workflows without failing when fork notes are absent.

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[CLAassistant]

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