feat(scheduler-agent): Phase 1 write operations — submit/cancel/hold/release + templates

Add write operations to the scheduler-agent MCP server (Phase 1):

- schedulerclient: SubmitJob/CancelJob/HoldJob/ReleaseJob + JobSpec/
  SubmitJobResult/JobControlRequest/ControlResult types + doPost helper
  (HTTP 200 + envelope-code dual check). TDD: 12 new tests.
- mcp-server: 4 write tools (scheduler_submit/cancel/hold/release) with
  destructive annotations; scheduler_submit has dry_run review mode +
  general_serial (通用串行脚本) job template registry. Fix the 6 read-only
  tools' misleading destructive annotations. TDD: 14 new tests.

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

src/frontend/schedulerclient/client.go

@@ -4,7 +4,7 @@ import "context"
 
 // ── Client interface ────────────────────────────────────────────────────────
 
-// Client provides read-only access to the scheduler cluster through a
+// Client provides read and write access to the scheduler cluster through a
 // scheduler-agnostic unified resource model (URM).
 //
 // Implementations talk to the apiserver's REST API and authenticate via
@@ -36,4 +36,18 @@ type Client interface {
 
 	// GetClusterMetrics returns a snapshot of overall cluster resource usage.
 	GetClusterMetrics(ctx context.Context) (*Metrics, error)
+
+	// ── Write operations (Phase 1) ──
+
+	// SubmitJob submits a job and returns its assigned ID.
+	SubmitJob(ctx context.Context, spec JobSpec) (*SubmitJobResult, error)
+
+	// CancelJob kills a job (action "kill", default SIGTERM).
+	CancelJob(ctx context.Context, id string) error
+
+	// HoldJob suspends a job (action "stop", default SIGTSTP).
+	HoldJob(ctx context.Context, id string) error
+
+	// ReleaseJob resumes a suspended job (action "resume", default SIGCONT).
+	ReleaseJob(ctx context.Context, id string) error
 }