docs(revert-rescue): record file-only rescue scope for non-file schemes

[mtskf]

Summary

Investigation outcome for the round-2 review finding that the revert-rescue is skipped on editable non-file schemes. Records the file-only rescue scope with rationale (documentation route) rather than widening the gate. No runtime behavior change.

Changes

• Expanded the RescueContext.canWrite / AliveRevertContext.canWrite gate doc in src/extension/surface/revert-rescue.ts to explain why the rescue is intentionally scoped to file: documents (unified with the reducer's canHostWrite write gate), not widened to every open-and-applyEdit-capable scheme.
• Recorded the full investigation + written-decision + re-visit trigger in .claude/docs/LEARNING.md (local-only doc; .claude/ is git-ignored in this repo).

Rationale (why documentation, not widening)

• The headline scenario (vscode-vfs / github.dev / remote virtual workspace) is unreachable: package.json declares capabilities.virtualWorkspaces.supported = false + untrustedWorkspaces.supported = false, so VS Code never activates Quoll in a virtual/untrusted workspace.
• The quoll.editWith gate (canEditWith) already refuses non-file schemes; the only residual entry is a forced vscode.openWith / hot-exit restore in a trusted local workspace with a concurrently-open dirty text editor on a writable non-file FS — a corner of a corner.
• Widening would fork the write-capability gate for a manifest-excluded case and create a "refuses normal writes but performs rescue writes" asymmetry on a security-sensitive write path, breaking the single-allowlist sync file-system.ts maintains between the command gate and the write gate.

Related

• TODO: 🐛 [LOW] Investigate: revert-rescue skipped on editable non-file schemes (canHostWrite = file-only)

Test Plan

• pnpm compile clean
• pnpm test:unit green (composition already pinned: can-host-write.test.ts refuses vscode-vfs even when FS is writable; revert-rescue.test.ts pins canWrite:false → {rescue:false} on both decision paths)
• pnpm lint clean on the changed file