feat(projman): add checkpoint/resume for interrupted agent work (#237)

Executor checkpointing:
- Standard checkpoint comment format with branch, commit, phase
- Files modified with status (created, modified)
- Completed and pending steps tracking
- State notes for resumption context
- Save checkpoint after major steps, before stopping

Orchestrator resume detection:
- Scan issue comments for "## Checkpoint" markers
- Offer resume options: resume, start fresh, review details
- Verify branch exists and files match before resuming
- Dispatch executor with checkpoint context

Sprint-start integration:
- Checkpoint detection as first workflow step
- Resume flow documentation with example
- Checkpoint format specification

This enables resuming work after:
- Budget exhaustion (100 tool call limit)
- Agent failure/circuit breaker
- Manual interruption
- Session timeout

Closes #237

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/projman/agents/orchestrator.md

@@ -93,7 +93,44 @@ git branch --show-current
 
 **Workflow:**
 
-**A. Fetch Sprint Issues**
+**A. Fetch Sprint Issues and Detect Checkpoints**
+```
+list_issues(state="open", labels=["sprint-current"])
+```
+
+**For each open issue, check for checkpoint comments:**
+```
+get_issue(issue_number=45)  # Comments included
+→ Look for comments containing "## Checkpoint"
+```
+
+**If Checkpoint Found:**
+```
+Checkpoint Detected for #45
+
+Found checkpoint from previous session:
+  Branch: feat/45-jwt-service
+  Phase: Testing
+  Tool Calls: 67
+  Files Modified: 3
+  Completed: 4/7 steps
+
+Options:
+1. Resume from checkpoint (recommended)
+2. Start fresh (discard previous work)
+3. Review checkpoint details first
+
+Would you like to resume?
+```
+
+**Resume Protocol:**
+1. Verify branch exists: `git branch -a | grep feat/45-jwt-service`
+2. Switch to branch: `git checkout feat/45-jwt-service`
+3. Verify files match checkpoint
+4. Dispatch executor with checkpoint context
+5. Executor continues from pending steps
+
+**B. Fetch Sprint Issues (Standard)**
 ```
 list_issues(state="open", labels=["sprint-current"])
 ```