fix(ax): make JSON state writes atomic

Co-Authored-By: Virgil <virgil@lethean.io>

.core/reference/docs/RFC.md

@@ -423,9 +423,10 @@ Every exported function MUST have a usage-example comment:
 
 ## Changelog
 
-- 2026-03-29: cmd/core-agent no longer rewrites `os.Args` before startup. The binary-owned commands now use named handlers, keeping the entrypoint on Core CLI primitives instead of repo-local argument mutation.
+- 2026-03-30: plan files and review queue rate-limit state now use `WriteAtomic`, keeping JSON state writes aligned with the AX safe-write convention.
 - 2026-03-30: transport helpers preserve request and read causes, brain direct API calls surface upstream bodies, and review queue retry parsing no longer uses `MustCompile`.
 - 2026-03-30: direct Core process calls replaced the `proc.go` wrapper layer; PID helpers now live in `pid.go` and the workspace template documents `c.Process()` directly.
+- 2026-03-29: cmd/core-agent no longer rewrites `os.Args` before startup. The binary-owned commands now use named handlers, keeping the entrypoint on Core CLI primitives instead of repo-local argument mutation.
 - 2026-03-26: net/http consolidated to transport.go (ONE file). net/url + io/fs eliminated. RFC-025 updated with 3 new quality gates (net/http, net/url, io/fs). 1:1 test + example test coverage. Array[T].Deduplicate replaces custom helpers.
 - 2026-03-25: Quality gates pass. Zero disallowed imports (all 10). encoding/json→Core JSON. path/filepath→Core Path. os→Core Env/Fs. io→Core ReadAll/WriteAll. go-process fully Result-native. ServiceRuntime on all subsystems. 22 named Actions + Task pipeline. ChannelNotifier→IPC. Reference docs synced.
 - 2026-03-25: Initial spec — written with full core/go v0.8.0 domain context.

docs/RFC.md

@@ -423,6 +423,7 @@ Every exported function MUST have a usage-example comment:
 
 ## Changelog
 
+- 2026-03-30: plan files and review queue rate-limit state now use `WriteAtomic`, keeping JSON state writes aligned with the AX safe-write convention.
 - 2026-03-30: plan create tests now assert the documented `core.ID()` shape and repeated plan creation produces unique IDs, keeping the plan contract aligned with the simplified generator.
 - 2026-03-30: dispatch completion monitoring now uses a named helper instead of an inline Action closure, keeping the spawned-process finaliser AX-native.
 - 2026-03-30: lib task bundle and recursive embed traversal now use `JoinPath` for filesystem paths, removing the last string-concatenated path joins in `pkg/lib`.

pkg/agentic/plan.go

@@ -425,7 +425,7 @@ func writePlanResult(dir string, plan *Plan) core.Result {
 
 	path := planPath(dir, plan.ID)
 
-	if r := fs.Write(path, core.JSONMarshalString(plan)); !r.OK {
+	if r := fs.WriteAtomic(path, core.JSONMarshalString(plan)); !r.OK {
 		err, _ := r.Value.(error)
 		if err == nil {
 			return core.Result{Value: core.E("writePlan", "failed to write plan", nil), OK: false}

pkg/agentic/review_queue.go

@@ -368,7 +368,7 @@ func (s *PrepSubsystem) storeReviewOutput(repoDir, repo, reviewer, output string
 // saveRateLimitState persists rate limit info for cross-run awareness.
 func (s *PrepSubsystem) saveRateLimitState(info *RateLimitInfo) {
 	path := core.JoinPath(core.Env("DIR_HOME"), ".core", "coderabbit-ratelimit.json")
-	fs.Write(path, core.JSONMarshalString(info))
+	fs.WriteAtomic(path, core.JSONMarshalString(info))
 }
 
 // loadRateLimitState reads persisted rate limit info.