fix(mcp): anchor PR context at merge base

[ScriptedAlchemy]

Summary

• make pr_context use true three-dot PR semantics by diffing the merge base against the head
• enumerate commits reachable from head but hidden by the merge base instead of walking first-parent until the moving base tip
• detect the repository default branch when base_ref is omitted
• expose the resolved merge-base hash in the result

Dogfood failure

Reviewing #389 from a branch whose origin/master had advanced made tracedecay tool pr_context report 299 files and up to 100 unrelated historical commits for a two-file PR. The same defect inflated another two-file review to 11 files. Raw git diff origin/master...HEAD showed the correct scope.

Tests

• merge-base regression with a diverged feature branch: pass
• git handler unit tests: 5 passed
• MCP pr_context integration tests: 2 passed
• cargo clippy --workspace --all-targets --locked -- -D warnings: pass

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Latest commit: b624387

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Prefer the fetched remote default for omitted bases

When origin/HEAD is present, branch::detect_default_branch() strips refs/remotes/origin/ and returns only master/main. In repositories where the local default branch is stale but origin/<default> has been fetched, this omitted-base path passes the stale local ref into git_pr_comparison, so the merge base can move back before the PR fork point and pr_context again includes unrelated default-branch changes. Prefer the remote ref when that is what was detected, or otherwise verify the local ref is current.

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