Optimize hooks-mode cherry-pick finalization performance

[svarlamov]

Summary

• align cherry-pick hooks-mode rewrite finalization with rebase-style terminal-only finalization
• tighten cherry-pick terminal-step detection so heavy rewrite runs once at sequence end
• skip managed repo lookup for prepare-commit-msg to remove per-commit overhead in hooks mode
• add regression assertion that multi-commit cherry-pick emits exactly one cherry_pick_complete event with full mappings

Validation

• cargo fmt -- --check
• cargo clippy
• cargo test -- --test-threads=8
• cargo test --test hook_modes -- --test-threads=8
• python3 scripts/benchmarks/git/benchmark_modes_vs_main.py --iterations-basic 5 --iterations-complex 4 --margin-pct 25.0 --margin-baseline current_wrapper --enforce-margin --work-root /tmp/git-ai-bench-after-full-1771956806
• python3 scripts/benchmarks/git/benchmark_nasty_modes_vs_main.py --work-root /tmp/git-ai-nasty-pr-smoke-1771960740 --feature-commits 24 --main-commits 10 --side-commits 8 --files 4 --lines-per-file 400 --burst-every 6 --repetitions 1 --margin-pct 25 --margin-baseline current_wrapper --enforce-margin

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