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
• remove every historical path in both batched note writers before writing the canonical path
• teach batched note reads to recognize every fanout depth with deterministic flat/canonical/deeper 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
• task test
• task lint
• task fmt

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[svarlamov]

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

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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