fix(gsd): fail-closed stop guard, harden backtrack parsing, fix prompt params

- Stop/backtrack guard now calls pauseAuto before marking captures executed,
  and returns break on any exception to prevent silently dropping user halt intent
- Backtrack target parsing excludes current milestone ID and rejects ambiguous
  multi-target strings instead of guessing first match
- Fixed gsd_skip_slice parameter names in rethink prompt (milestone_id → milestoneId)

src/resources/extensions/gsd/auto/phases.ts

@@ -721,6 +721,8 @@ export async function runGuards(
   // ── Stop/Backtrack directive guard (#3487) ──
   // Check for unexecuted stop or backtrack captures BEFORE dispatching any unit.
   // This ensures user "halt" directives are honored immediately.
+  // IMPORTANT: Fail-closed — any exception during stop handling still breaks the loop
+  // to ensure user halt intent is never silently dropped.
   try {
     const { loadStopCaptures, markCaptureExecuted } = await import("../captures.js");
     const stopCaptures = loadStopCaptures(s.basePath);
@@ -737,12 +739,10 @@ export async function runGuards(
         basename(s.originalBasePath || s.basePath),
       );
 
-      // Mark all stop/backtrack captures as executed so they don't re-fire
-      for (const cap of stopCaptures) {
-        markCaptureExecuted(s.basePath, cap.id);
-      }
+      // Pause first — ensures auto-mode stops even if later steps fail
+      await deps.pauseAuto(ctx, pi);
 
-      // For backtrack captures, write the backtrack trigger before pausing
+      // For backtrack captures, write the backtrack trigger after pausing
       if (isBacktrack) {
         try {
           const { executeBacktrack } = await import("../triage-resolution.js");
@@ -752,12 +752,19 @@ export async function runGuards(
         }
       }
 
-      await deps.pauseAuto(ctx, pi);
+      // Mark captures as executed only after successful pause/transition
+      for (const cap of stopCaptures) {
+        markCaptureExecuted(s.basePath, cap.id);
+      }
+
       debugLog("autoLoop", { phase: "exit", reason: isBacktrack ? "user-backtrack" : "user-stop" });
       return { action: "break", reason: isBacktrack ? "user-backtrack" : "user-stop" };
     }
   } catch (e) {
+    // Fail-closed: if anything in the stop guard throws, break the loop
+    // rather than silently continuing and dropping user halt intent
     debugLog("guards", { phase: "stop-guard-error", error: String(e) });
+    return { action: "break", reason: "stop-guard-error" };
   }
 
   // Budget ceiling guard

src/resources/extensions/gsd/prompts/rethink.md

@@ -48,7 +48,7 @@ Remove the `{ID}-PARKED.md` file from the milestone directory to reactivate it.
 ### Skip a slice
 Mark a slice as skipped so auto-mode advances past it without executing. Use the `gsd_skip_slice` tool:
 ```
-gsd_skip_slice({ milestone_id: "M003", slice_id: "S02", reason: "Descoped — feature moved to M005" })
+gsd_skip_slice({ milestoneId: "M003", sliceId: "S02", reason: "Descoped — feature moved to M005" })
 ```
 Skipped slices are treated as closed by the state machine (like "complete" but distinct). Use when a slice is no longer needed or has been superseded. The slice data is preserved for reference.
 

src/resources/extensions/gsd/triage-resolution.ts

@@ -148,10 +148,17 @@ export function executeBacktrack(
   capture: CaptureEntry,
 ): string | null {
   try {
-    // Extract target milestone from capture text or resolution
-    const targetMatch = (capture.resolution ?? capture.text)
-      .match(/\b(M\d{3}(?:-[a-z0-9]{6})?)\b/);
-    const targetMilestoneId = targetMatch?.[1] ?? null;
+    // Extract target milestone from capture text or resolution.
+    // Filter out the current milestone ID to avoid picking it as the backtrack target
+    // when the text mentions both current and target milestones (e.g. "backtrack from M004 to M003").
+    const sourceText = capture.resolution ?? capture.text;
+    const allMatches = [...sourceText.matchAll(/\b(M\d{3}(?:-[a-z0-9]{6})?)\b/g)]
+      .map(m => m[1])
+      .filter(id => id !== currentMilestoneId);
+    // Reject ambiguous multi-target strings — if more than one distinct target remains,
+    // don't guess; let the user clarify.
+    const uniqueTargets = [...new Set(allMatches)];
+    const targetMilestoneId = uniqueTargets.length === 1 ? uniqueTargets[0] : null;
 
     const ts = new Date().toISOString();
     const triggerPath = join(gsdRoot(basePath), "BACKTRACK-TRIGGER.md");