docs: refresh docs to reflect edit_replace additions

[clouatre]

Summary

Refresh four documentation files to reflect recent edit_replace enhancements merged in PRs #1380 and #1381. The tool now supports an optional expected_content_hash (blake3) staleness check, serializes concurrent edits to the same file via a per-path mutex, and returns a new stale_content_hash error subtype. These docs-only changes keep AGENTS.md, README.md, METRICS.md, and ROADMAP.md current with the implementation.

Changes

The README.md edit_replace table row now mentions the optional expected_content_hash parameter and per-path concurrent edit serialization. AGENTS.md adds a new bullet in the tool parameters section documenting the staleness check and mutex registry behavior, placed before the existing circuit-breaker bullet. The METRICS.md schema table and JSON example block both add stale_content_hash to the error_subtype examples. ROADMAP.md appends a new completed entry below the Knowledge Graph entry summarizing the concurrent safety, staleness detection, and hot-path optimizations delivered in #1380 and #1381.

Test plan

• Tests pass (cargo fmt and cargo clippy clean; no Rust source changes)
• Linter clean (cargo fmt --check, cargo clippy -- -D warnings)
• Security scan clean (no findings in diff)