fix(freebuff): derive commit summaries from git changes

[victorxheng]

User request

A Freebuff cloud user reported that commits shown in the Git tab after pushing use the original prompt text as the commit description instead of a meaningful summary of the actual project changes. The attached screenshot showed prompt-derived commit titles such as Freebuff: Add this below the AI instructions... and Before: When committing changes....

Implementation

• Added summarizeGitChangesForCommit, a deterministic shared helper that builds concise commit titles from git status/diff paths instead of user prompt text.
• Covered the reported case with a unit test: modified AGENTS.md plus new CHANGELOG.md now summarizes as Update AI agent instructions and add changelog.
• Strengthened the commit tool guide so agent-created commits explicitly base title/body on actual git status/diff and never copy/truncate the original prompt.
• Updated the example git-committer agent instructions to follow the same rule.

Validation

• bun test common/src/util/__tests__/git-commit-summary.test.ts
• cd common && bun run typecheck
• git diff --check

Confidence

Ready for review. The exact cloud Git-tab route is not present in this public snapshot, so this PR places the fix in shared code and the commit-generation contract that cloud/private code can call or inherit. The covered behavior directly matches the user report and prevents prompt-only summaries in the shared agent commit path.