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/api/routes/issues.py

@@ -31,7 +31,7 @@ from app.crud.syndarix.agent_instance import agent_instance as agent_instance_cr
 from app.crud.syndarix.issue import issue as issue_crud
 from app.crud.syndarix.project import project as project_crud
 from app.crud.syndarix.sprint import sprint as sprint_crud
-from app.models.syndarix.enums import IssuePriority, IssueStatus, SyncStatus
+from app.models.syndarix.enums import AgentStatus, IssuePriority, IssueStatus, SyncStatus
 from app.models.user import User
 from app.schemas.common import (
     MessageResponse,
@@ -200,6 +200,12 @@ async def create_issue(
                 error_code=ErrorCode.VALIDATION_ERROR,
                 field="assigned_agent_id",
             )
+        if agent.status == AgentStatus.TERMINATED:
+            raise ValidationException(
+                message="Cannot assign issue to a terminated agent",
+                error_code=ErrorCode.VALIDATION_ERROR,
+                field="assigned_agent_id",
+            )
 
     # Validate sprint if provided (IDOR prevention)
     if issue_in.sprint_id:
@@ -537,6 +543,12 @@ async def update_issue(
                 error_code=ErrorCode.VALIDATION_ERROR,
                 field="assigned_agent_id",
             )
+        if agent.status == AgentStatus.TERMINATED:
+            raise ValidationException(
+                message="Cannot assign issue to a terminated agent",
+                error_code=ErrorCode.VALIDATION_ERROR,
+                field="assigned_agent_id",
+            )
 
     # Validate sprint if being updated (IDOR prevention)
     if issue_in.sprint_id is not None:
@@ -730,6 +742,12 @@ async def assign_issue(
                 error_code=ErrorCode.VALIDATION_ERROR,
                 field="assigned_agent_id",
             )
+        if agent.status == AgentStatus.TERMINATED:
+            raise ValidationException(
+                message="Cannot assign issue to a terminated agent",
+                error_code=ErrorCode.VALIDATION_ERROR,
+                field="assigned_agent_id",
+            )
 
         updated_issue = await issue_crud.assign_to_agent(
             db, issue_id=issue_id, agent_id=assignment.assigned_agent_id