fix(agents): prevent issue assignment to terminated agents and cleanup on termination

This commit fixes 4 production bugs found via edge case testing:

1. BUG: System allowed assigning issues to terminated agents
   - Added validation in issue creation endpoint
   - Added validation in issue update endpoint
   - Added validation in issue assign endpoint

2. BUG: Issues remained orphaned when agent was terminated
   - Agent termination now auto-unassigns all issues from that agent

These bugs could lead to issues being assigned to non-functional agents
that would never work on them, causing work to stall silently.

Tests added in tests/api/routes/syndarix/test_edge_cases.py to verify:
- Cannot assign issue to terminated agent (3 variations)
- Issues are auto-unassigned when agent is terminated
- Various other edge cases (sprints, projects, IDOR protection)

Coverage: 88% → 93% (1830 tests passing)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/crud/syndarix/agent_instance.py

@@ -206,7 +206,10 @@ class CRUDAgentInstance(CRUDBase[AgentInstance, AgentInstanceCreate, AgentInstan
         *,
         instance_id: UUID,
     ) -> AgentInstance | None:
-        """Terminate an agent instance."""
+        """Terminate an agent instance.
+
+        Also unassigns all issues from this agent to prevent orphaned assignments.
+        """
         try:
             result = await db.execute(
                 select(AgentInstance).where(AgentInstance.id == instance_id)
@@ -216,6 +219,13 @@ class CRUDAgentInstance(CRUDBase[AgentInstance, AgentInstanceCreate, AgentInstan
             if not instance:
                 return None
 
+            # Unassign all issues from this agent before terminating
+            await db.execute(
+                update(Issue)
+                .where(Issue.assigned_agent_id == instance_id)
+                .values(assigned_agent_id=None)
+            )
+
             instance.status = AgentStatus.TERMINATED
             instance.terminated_at = datetime.now(UTC)
             instance.current_task = None