fix(agent-status): prevalidate subagent ids before allocating a roster

Review finding round 2: upsert silently rejects over-length agent ids after
the caller already allocated, stranding one empty roster per invented pane
key with zero emitted payloads. Export the id validity check and gate both
wire-reachable allocation sites.

src/shared/agent-hook-listener.test.ts

@@ -3113,6 +3113,25 @@ describe('shared agent-hook-listener', () => {
       expect(state.claudeSubagentRosterByPaneKey.has(PANE_KEY)).toBe(false)
     })
 
+    it('does not retain a roster when a child tool event carries an id the upsert rejects', () => {
+      // Why: upsert silently drops over-length ids; allocating first would strand
+      // an empty roster per invented pane key with zero emitted payloads.
+      const childTool = claudeEvent({
+        hook_event_name: 'PreToolUse',
+        agent_id: 'a'.repeat(65),
+        tool_name: 'Bash',
+        tool_input: { command: 'true' }
+      })
+      expect(childTool).toBeNull()
+      expect(state.claudeSubagentRosterByPaneKey.has(PANE_KEY)).toBe(false)
+    })
+
+    it('does not retain a roster for a SubagentStart with an id the upsert rejects', () => {
+      const spawned = claudeEvent({ hook_event_name: 'SubagentStart', agent_id: 'a'.repeat(65) })
+      expect(spawned).toBeNull()
+      expect(state.claudeSubagentRosterByPaneKey.has(PANE_KEY)).toBe(false)
+    })
+
     it('re-emits the lead done state on a start-less SubagentStop after a completed turn', () => {
       claudeEvent({ hook_event_name: 'UserPromptSubmit', prompt: 'go' })
       claudeEvent({ hook_event_name: 'Stop', background_tasks: [] })

src/shared/agent-hook-listener.ts

@@ -33,6 +33,7 @@ import {
   foldClaudeBackgroundTasksIntoRoster,
   hasActiveClaudeNonAgentBackgroundWork,
   idleClaudeTeammateByName,
+  isValidClaudeSubagentId,
   readClaudeBackgroundAgentTasks,
   reapRestoredClaudeSubagentsWithoutLiveAgent,
   stopClaudeSubagent,
@@ -2446,13 +2447,16 @@ function normalizeClaudeSubagentLifecycleEvent(
   } else {
     const agentId = lifecycleId
     if (eventName === 'SubagentStart') {
-      upsertWorkingClaudeSubagent(
-        getOrCreateClaudeSubagentRoster(state, paneKey),
-        agentId,
-        { agentType: readString(hookPayload, 'agent_type') },
-        Date.now()
-      )
-      rosterChanged = true
+      // Why: upsert rejects invalid ids silently — allocate only for an id it will accept.
+      if (isValidClaudeSubagentId(agentId)) {
+        upsertWorkingClaudeSubagent(
+          getOrCreateClaudeSubagentRoster(state, paneKey),
+          agentId,
+          { agentType: readString(hookPayload, 'agent_type') },
+          Date.now()
+        )
+        rosterChanged = true
+      }
     } else {
       if (roster) {
         // Why: a stop of an untracked id (e.g. compact's start-less SubagentStop) is a roster no-op, not evidence.
@@ -2631,7 +2635,12 @@ function normalizeClaudeEvent(
       eventName === 'PostToolUseFailure')
       ? eventAgentId
       : undefined
-  if (eventAgentId && (subagentOriginId || isWaitingInducing)) {
+  if (
+    eventAgentId &&
+    (subagentOriginId || isWaitingInducing) &&
+    isValidClaudeSubagentId(eventAgentId)
+  ) {
+    // Why: upsert rejects invalid ids silently — allocate only for an id it will accept.
     upsertWorkingClaudeSubagent(
       getOrCreateClaudeSubagentRoster(state, paneKey),
       eventAgentId,

src/shared/claude-subagent-roster.ts

@@ -73,6 +73,12 @@ export function isClaudeTeammateLifecycleId(id: string): boolean {
   return separator > 1 && id.startsWith('a') && /^[0-9a-f]+$/i.test(id.slice(separator + 1))
 }
 
+/** Pre-check for callers that allocate a roster to upsert into — upsert rejects
+ *  invalid ids silently, which would strand the fresh roster empty forever. */
+export function isValidClaudeSubagentId(id: string): boolean {
+  return id.length > 0 && id.length <= CLAUDE_SUBAGENT_ID_MAX_LENGTH
+}
+
 export function upsertWorkingClaudeSubagent(
   roster: ClaudeSubagentRoster,
   id: string,