feat(10): add /gsd:status for parallel agent monitoring

Simple command to check background agent status:
- Shows running/completed agents from agent-history.json
- Uses TaskOutput to poll agent status
- --wait flag blocks until all complete
- Reports time savings vs sequential execution

Inspired by PR #43 but intentionally minimal (117 lines vs 317).
Skipped task-level parallelization display (not implementing that).

commands/gsd/status.md

@@ -0,0 +1,161 @@
+---
+name: gsd:status
+description: Check status of background agents from parallel execution
+argument-hint: "[--wait]"
+allowed-tools:
+  - Read
+  - Write
+  - Bash
+  - TaskOutput
+---
+
+<objective>
+Monitor background agent status from /gsd:execute-phase parallel execution.
+
+Shows running/completed agents from agent-history.json.
+Uses TaskOutput to check status of background tasks.
+With --wait flag, blocks until all agents complete.
+</objective>
+
+<context>
+Arguments: $ARGUMENTS
+</context>
+
+<process>
+
+<step name="load_history">
+**Load agent history:**
+
+```bash
+cat .planning/agent-history.json 2>/dev/null || echo '{"entries":[]}'
+```
+
+If file doesn't exist or has no entries:
+```
+No background agents tracked.
+
+Run /gsd:execute-phase to spawn parallel agents.
+```
+Exit.
+</step>
+
+<step name="filter_agents">
+**Find background agents:**
+
+Filter entries where:
+- `execution_mode` is "parallel" or "background"
+- `status` is "spawned" (still running) or recently completed
+
+Group by `parallel_group` if present.
+</step>
+
+<step name="check_running">
+**Check status of running agents:**
+
+For each agent with `status === "spawned"`:
+
+Use TaskOutput tool:
+```
+task_id: [agent_id]
+block: false
+timeout: 1000
+```
+
+**If TaskOutput returns completed result:**
+- Update agent-history.json: status → "completed"
+- Set completion_timestamp
+- Parse files_modified from output if present
+
+**If TaskOutput returns "still running":**
+- Keep as spawned (running)
+
+**If TaskOutput returns error:**
+- Update agent-history.json: status → "failed"
+</step>
+
+<step name="display">
+**Show status table:**
+
+```
+Background Agents
+════════════════════════════════════════
+
+| Plan   | Status      | Elapsed  | Agent ID      |
+|--------|-------------|----------|---------------|
+| 10-01  | ✓ Complete  | 2m 15s   | agent_01H...  |
+| 10-02  | ⏳ Running   | 1m 30s   | agent_01H...  |
+| 10-04  | ✓ Complete  | 1m 45s   | agent_01H...  |
+
+Progress: 2/3 complete
+
+════════════════════════════════════════
+Wait for all: /gsd:status --wait
+```
+
+**Status icons:**
+- ✓ Complete
+- ⏳ Running
+- ✗ Failed
+- ⏸ Queued (waiting for dependency)
+</step>
+
+<step name="wait_mode">
+**If --wait flag provided:**
+
+For each agent with status "spawned":
+
+Use TaskOutput with blocking:
+```
+task_id: [agent_id]
+block: true
+timeout: 600000
+```
+
+Report as each completes:
+```
+⏳ Waiting for 3 agents...
+
+✓ [1/3] 10-01 complete (2m 15s)
+✓ [2/3] 10-04 complete (1m 45s)
+✓ [3/3] 10-02 complete (3m 30s)
+
+════════════════════════════════════════
+All agents complete!
+
+Total time: 3m 30s (parallel)
+Sequential estimate: 7m 30s
+Time saved: ~4m (53%)
+════════════════════════════════════════
+```
+
+Update agent-history.json with completion status for each.
+</step>
+
+<step name="next_steps">
+**After all complete (or if already complete):**
+
+```
+---
+
+## ▶ Next Up
+
+All parallel agents finished. Review results:
+
+`/gsd:progress`
+
+<sub>`/clear` first → fresh context window</sub>
+
+---
+```
+</step>
+
+</process>
+
+<success_criteria>
+- [ ] Reads agent-history.json for background agents
+- [ ] Uses TaskOutput to check running agent status
+- [ ] Updates history with current status
+- [ ] Shows simple status table
+- [ ] --wait flag blocks until all complete
+- [ ] Reports time savings vs sequential
+</success_criteria>