feat: add TenantCtx for compile-time tenant isolation

Implements zmanian's architectural proposal from #1614 review:
two-tier scoped database access (TenantScope/AdminScope) so handler
code cannot accidentally bypass tenant scoping.

TenantScope (default): wraps user_id + Arc<dyn Database>, auto-binds
user_id on every operation. ID-based lookups return None for cross-
tenant resources. No escape hatch — forgetting to scope is a compile
error.

AdminScope (explicit opt-in): cross-tenant access for system-level
components (heartbeat, routine engine, self-repair, scheduler, worker).

TenantCtx bundles TenantScope + workspace + cost guard + per-user
rate limiting. Constructed once per request in handle_message, threaded
through all command handlers and ChatDelegate.

Key changes:
- New src/tenant.rs (~920 lines): TenantScope, AdminScope, TenantCtx,
  TenantRateState, TenantRateRegistry
- All command handlers: user_id: &str → ctx: &TenantCtx
- ChatDelegate: cost check/record/settings via self.tenant
- System components: store field changed to AdminScope
- Config: TENANT_MAX_LLM_CONCURRENT, TENANT_MAX_JOBS_CONCURRENT env vars
- Fixes bug: /status <job_id> cross-tenant leak (now auto-filtered)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/agent/thread_ops.rs

@@ -175,6 +175,7 @@ impl Agent {
     pub(super) async fn process_user_input(
         &self,
         message: &IncomingMessage,
+        tenant: crate::tenant::TenantCtx,
         session: Arc<Mutex<Session>>,
         thread_id: Uuid,
         content: &str,
@@ -351,7 +352,7 @@ impl Agent {
 
         if let Some(intent) = self.router.route_command(&temp_message) {
             // Explicit command like /status, /job, /list - handle directly
-            return self.handle_job_or_command(intent, message).await;
+            return self.handle_job_or_command(intent, message, &tenant).await;
         }
 
         // Natural language goes through the agentic loop
@@ -462,7 +463,7 @@ impl Agent {
 
         // Run the agentic tool execution loop
         let result = self
-            .run_agentic_loop(message, session.clone(), thread_id, turn_messages)
+            .run_agentic_loop(message, tenant, session.clone(), thread_id, turn_messages)
             .await;
 
         // Re-acquire lock and check if interrupted
@@ -1444,7 +1445,13 @@ impl Agent {
 
             // Continue the agentic loop (a tool was already executed this turn)
             let result = self
-                .run_agentic_loop(message, session.clone(), thread_id, context_messages)
+                .run_agentic_loop(
+                    message,
+                    self.tenant_ctx(&message.user_id).await,
+                    session.clone(),
+                    thread_id,
+                    context_messages,
+                )
                 .await;
 
             // Handle the result