feat(M31): agent work routing — scope jobs to assigned agents

Deployment jobs now set agent_id from target→agent relationship at
creation time. GetPendingWork() uses ListPendingByAgentID() with a
3-way UNION query (direct match, legacy NULL fallback via target JOIN,
AwaitingCSR via cert→target→agent chain) so each agent only receives
its own jobs.

- Added AgentID *string to Job domain struct
- Added agent_id to all job SQL queries (5 SELECTs, INSERT, UPDATE, scanJob)
- New ListPendingByAgentID() repository method
- Rewrote GetPendingWork() from ~25 lines to single scoped query
- 4 new Go tests (3 agent routing + 1 deployment agent_id)
- Frontend: agent_id/target_id on Job type

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/repository/interfaces.go

@@ -111,6 +111,8 @@ type JobRepository interface {
 	UpdateStatus(ctx context.Context, id string, status domain.JobStatus, errMsg string) error
 	// GetPendingJobs returns jobs not yet processed of a specific type.
 	GetPendingJobs(ctx context.Context, jobType domain.JobType) ([]*domain.Job, error)
+	// ListPendingByAgentID returns pending deployment jobs and AwaitingCSR jobs for a specific agent.
+	ListPendingByAgentID(ctx context.Context, agentID string) ([]*domain.Job, error)
 }
 
 // RenewalPolicyRepository defines operations for managing renewal policies.