Add agent restart lifecycle command

Extends the existing CheckIn RPC with a one-shot AgentCommand
(restart only -- stop/uninstall need real per-platform OS
service-manager integration and stay deliberately out of scope),
delivered at-most-once: cleared the instant it's handed to the agent
in a response, since a restarting agent's process is gone before it
could ever confirm receipt. On restart, the agent flushes whatever's
buffered, aborts its source task, and exits cleanly, relying entirely
on the host's own service manager to bring it back up.

Issuing a command is gated at RoleAdmin (stricter than config
editing's RoleEditor) and logged into the same audit_log table Phase
7's AI interactions use, via a new agent_command event type.

A real bug was found and fixed during live verification: the first
implementation tried to atomically read-and-clear pending_command in
a single INSERT...ON CONFLICT statement using a sibling CTE
referenced only from RETURNING, on the assumption that Postgres
evaluates every part of a WITH query against one pre-statement
snapshot. That's wrong specifically for FOR UPDATE, which always
reads the latest row version including one written earlier in the
same statement -- confirmed empirically (a restart command was
always coming back empty even when genuinely pending, so the agent
never received it). Fixed by splitting into two real, ordered
statements inside one explicit transaction.

See /docs/agent-management-design.md's "Lifecycle commands" section.

enterprise/cmd/enterprise-api/main.go

@@ -165,7 +165,11 @@ func main() {
 
 	queryHandler := queryapi.NewHandler(logger, registry, search, cfg.QueryTimeout, auditLogger, authorizer)
 	dashboardsHandler := dashboards.NewHandler(logger, dashboards.NewStore(pgPool), authorizer, rbacstore.NewDashboardPermissions(rbac))
-	agentsHandler := agents.NewHandler(logger, agents.NewStore(pgPool), authorizer)
+	// Reuses the same audit_writer-role pool/store auditLogger above
+	// writes through -- same append-only audit_log table, new
+	// event_type (see metadata/migrations/0039).
+	commandLogger := audit.NewAgentCommandLogger(audit.NewStore(auditPool), audit.SourceAPI)
+	agentsHandler := agents.NewHandler(logger, agents.NewStore(pgPool), authorizer, commandLogger)
 
 	mux := http.NewServeMux()
 	queryHandler.RegisterRoutes(mux) // also registers GET /healthz