Handle all Git notes fanout depths

[svarlamov]

Fixes #1517

Summary

• enumerate every valid two-hex Git notes fanout depth instead of hard-coding flat and one-directory paths
• normalize returned note paths in place and keep one borrowed lookup entry per requested commit, avoiding 20 SHA-1 or 32 SHA-256 path entries per commit
• remove every historical path in both batched note writers before writing the canonical path
• emit cleanup commands directly into the fast-import buffer without allocating a temporary String for every fanout depth
• preserve deterministic flat/canonical/deeper read precedence

Test coverage

• reproduces duplicate-note JSON deserialization failure across flat, three-component, and maximally deep fanout paths
• verifies both content-backed and blob-backed batch writers remove every legacy copy
• verifies batch reads find deeply fanned-out historical notes
• verifies in-place path normalization and allocation-free deletion command generation
• task test
• task lint
• task fmt

---

[svarlamov]

• Handle all Git notes fanout depths #1908 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.