fs-local: POSIX-only mode-bit assertions, document Windows DACL-inheritance semantics

Windows drives only the read-only attribute through chmod and reports
synthetic stat mode bits, so writeFileAtomic's mode arguments are inert
there; write-in-progress privacy comes from the staging dir (created in
the target's parent) inheriting the destination directory's DACL.
Production is deliberately unchanged -- the chmod calls are benign
no-ops and platform-guarding them out buys nothing. Tests guard the
mode-bit expects to POSIX; there is no Windows ACL assertion because an
ACL check would pin OS inheritance plus the machine's %TEMP% ACL, not
this package. Decision and rejected alternatives (explicit DACLs,
Get-Acl/icacls test verification) recorded in the new RFC.

docs/rfc/INDEX.md

@@ -150,6 +150,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 | [Prompt variables and tool-guidance ownership](implemented/architecture/2026-07-05-prompt-variables-and-tool-guidance-ownership.md) | 2026-07-05 |
 | [Every LLM request is reconstructable from the session log](implemented/architecture/2026-07-05-reconstructable-requests.md) | 2026-07-05 |
 | [Subagent provider-lifecycle events — `subagent/provider-added` / `subagent/provider-removed`](implemented/architecture/2026-07-05-subagent-provider-lifecycle-events.md) | 2026-07-05 |
+| [Windows write-permission semantics — inherited DACLs, not mode bits](implemented/architecture/2026-07-05-windows-fs-permissions.md) | 2026-07-05 |
 | [Windows-native durable JSONL publication](implemented/architecture/2026-07-05-windows-jsonl-durable-publish.md) | 2026-07-05 |
 | [A shared timeout/deadline primitive, with hard-kill left to each capability](implemented/architecture/2026-07-06-timeout-deadline-library.md) | 2026-07-06 |
 | [Tool result retention library](implemented/architecture/2026-07-06-tool-result-retention-library.md) | 2026-07-06 |