feat(impact): add git diff impact reports

[NicholaiVogel]

Summary

• add git-diff-aware impact analysis in graphiq-core
• expose graphiq impact for working-tree and base...head comparisons
• add MCP impact tool for agent triage reports with text/JSON output
• handle deletion-only hunks using old-side ranges so deleted files map all removed symbols

Test Plan

• cargo test -p graphiq-core impact -- --nocapture
• cargo fmt --all -- --check
• cargo check -p graphiq-cli -p graphiq-mcp
• cargo test -p graphiq-cli
• cargo test -p graphiq-mcp
• temp-repo smoke for working-tree impact
• temp-repo smoke for --base main impact
• temp-repo smoke for deleted-file impact mapping multiple removed symbols