fix(atlas): scope no-tool-progress counter to active plan path before stall

Oracle review on PR #4132 flagged that resetStallStateForPlanChange only
reset state when stalledContinuationReason was already set, so partial
in-progress counts from plan A could carry to plan B and abort the new
plan on its first idle.

Adds activeContinuationPlanPath to SessionState and resets the
no-tool-progress counter + awaiting state whenever the active plan path
changes, even when no stall has occurred yet. The existing stalled-plan
reset still fires when the previously stalled plan path differs from
the new one.

src/hooks/atlas/idle-event.ts

@@ -27,6 +27,7 @@ import { resolveActiveBoulderSession } from "./resolve-active-boulder-session"
 import { BOULDER_COMPLETE_PROMPT } from "./system-reminder-templates"
 import {
   markContinuationStalled,
+  resetStallStateForPlanChange,
   shouldAbortForNoToolProgress,
   updateNoToolProgressIterations,
 } from "./tool-progress"
@@ -352,6 +353,8 @@ export async function handleAtlasSessionIdle(input: {
   }
 
   const now = Date.now()
+  const activePlanPath = resolveBoulderPlanPath(ctx.directory, boulderState)
+  resetStallStateForPlanChange(sessionState, activePlanPath)
 
   if (sessionState.waitingForFinalWaveApproval) {
     log(`[${HOOK_NAME}] Skipped: waiting for explicit final-wave approval`, { sessionID })
@@ -368,7 +371,7 @@ export async function handleAtlasSessionIdle(input: {
 
   const noProgressIterations = updateNoToolProgressIterations(sessionState)
   if (shouldAbortForNoToolProgress(sessionState)) {
-    markContinuationStalled(sessionState, boulderState.plan_name)
+    markContinuationStalled(sessionState, boulderState.plan_name, activePlanPath)
     if (sessionState.pendingRetryTimer) {
       clearTimeout(sessionState.pendingRetryTimer)
       sessionState.pendingRetryTimer = undefined

src/hooks/atlas/index.test.ts

@@ -2015,6 +2015,54 @@ session_id: ses_untrusted_999
       }
     })
 
+    test("#given one plan stalls #when a different boulder plan becomes active #then Atlas continues the new plan", async () => {
+      // given - a boulder plan that reaches the stalled no-tool-progress threshold
+      const firstPlanPath = join(TEST_DIR, "first-blocked-loop-plan.md")
+      writeFileSync(firstPlanPath, "# Plan\n- [ ] Wait for external approval")
+
+      writeBoulderState(TEST_DIR, {
+        active_plan: firstPlanPath,
+        started_at: "2026-01-02T10:00:00Z",
+        session_ids: [MAIN_SESSION_ID],
+        plan_name: "first-blocked-loop-plan",
+      })
+
+      const mockInput = createMockPluginInput()
+      const hook = createTestAtlasHook(mockInput)
+
+      const originalDateNow = Date.now
+      let now = 0
+      Date.now = () => now
+
+      try {
+        for (let iteration = 0; iteration < 4; iteration += 1) {
+          await hook.handler({ event: { type: "session.idle", properties: { sessionID: MAIN_SESSION_ID } } })
+          await flushMicrotasks()
+          now += 6000
+        }
+        expect(mockInput._promptMock).toHaveBeenCalledTimes(3)
+
+        const secondPlanPath = join(TEST_DIR, "second-plan.md")
+        writeFileSync(secondPlanPath, "# Plan\n- [ ] Fresh task")
+        writeBoulderState(TEST_DIR, {
+          active_plan: secondPlanPath,
+          started_at: "2026-01-02T10:10:00Z",
+          session_ids: [MAIN_SESSION_ID],
+          plan_name: "second-plan",
+        })
+        now += 6000
+
+        // when - the same session id receives a different active plan
+        await hook.handler({ event: { type: "session.idle", properties: { sessionID: MAIN_SESSION_ID } } })
+        await flushMicrotasks()
+
+        // then - stale stall state from the previous plan does not permanently block continuation
+        expect(mockInput._promptMock).toHaveBeenCalledTimes(4)
+      } finally {
+        Date.now = originalDateNow
+      }
+    })
+
     test("#given continuation makes tangible tool progress #when idle repeats #then no-progress stall counter resets", async () => {
       // given - boulder state with incomplete work and a successful edit between continuation turns
       const planPath = join(TEST_DIR, "progress-plan.md")