feat(tool): add hashline_edit tool and complete registry wiring

[randomm]

Summary

• Adds hashline_edit tool (src/tool/hashline_edit.ts) — applies precise line edits using content-addressable anchors from hashline_read
• Adds tool description (hashline_edit.txt)
• Completes registry wiring: HashlineEditTool added to all(), standard edit tool suppressed in tools() when OPENCODE_EXPERIMENTAL_HASHLINE is enabled
• 10 tests covering all edit ops, error handling, atomicity, and flag gating

Part of hashline EPIC #209. Depends on Phase 1 (#210, PR #215) and Phase 2 (#211, PR #216).

Closes #212

Key implementation details

• Uses applyHashlineEdits() from hashline.ts for all edit operations
• HashlineMismatchError.message already includes →-marked current lines — returned directly to model
• FileTime.withLock ensures safe concurrent access
• Registry: tools() filter suppresses EditTool when hashline flag is active

Quality Gates

• Adversarial review: APPROVED
• 10 tests pass locally
• Typecheck: 0 errors
• No as any, @ts-ignore, or suppressions

[randomm]

Pre-review fixes applied:

• Removed redundant try/catch in hashline_edit.ts (all branches rethrew anyway)
• Removed unused HashlineMismatchError/HashlineNoOpError imports
• Moved let declarations inside lock closure as const
• Fixed registry.ts: write tool was incorrectly suppressed alongside edit when hashline flag is active; now only edit is suppressed