feat: add Safe Exit protocol for blocked tasks under new task file convention

Agents no longer commit task files, so when an agent safe-exits:
- Agent notifies coordinator with detailed blocking message (what happened, what was tried, suggested fix)
- Agent commits any partial source code progress and pushes
- Coordinator updates task file on main with status: blocked and the reason
- Coordinator attempts resolution, spawns new agent if resolved

Also removed stale 'task file conflicts' from merge workflow since
agents no longer commit task files.

.opencode/agents/implementation-specialist.md

@@ -149,11 +149,17 @@ When task becomes untendable:
 ### Process
 
 1. **Stop** - don't force through
-2. **Notify coordinator**:
+2. **Notify coordinator** with a detailed blocking message. Include:
+   - What you were trying to do
+   - What went wrong (specific error, missing dep, ambiguous spec, etc.)
+   - What you've already tried
+   - What you think would resolve it (if you know)
    ```text
-   worktree({action: "notify", args: {message: "Blocked on <task-id>: <reason>", level: "blocking"}})
+   worktree({action: "notify", args: {message: "Blocked on <task-id>: <detailed explanation including what was attempted, what failed, and suggested resolution>", level: "blocking"}})
    ```
-3. **Exit** - coordinator handles escalation
+3. **Commit any partial source code progress** if it's coherent (you may not have any — that's fine)
+4. **Push your branch** so the coordinator can inspect your work if needed
+5. **Exit** - coordinator handles escalation
 
 ### Wrong Directory Recovery