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]>

tests/support/gateway_workflow_harness.rs

@@ -265,6 +265,7 @@ impl GatewayWorkflowHarness {
                 sandbox_readiness: ironclaw::agent::SandboxReadiness::DisabledByConfig,
                 builder: None,
                 llm_backend: "nearai".to_string(),
+                tenant_rates: std::sync::Arc::new(ironclaw::tenant::TenantRateRegistry::new(4, 3)),
             },
             channels,
             None,

tests/support/test_rig.rs

@@ -642,7 +642,7 @@ impl TestRigBuilder {
                 let (notify_tx, _notify_rx) = tokio::sync::mpsc::channel(16);
                 let engine = Arc::new(RoutineEngine::new(
                     routine_config,
-                    Arc::clone(db_arc),
+                    ironclaw::tenant::AdminScope::new(Arc::clone(db_arc)),
                     components.llm.clone(),
                     Arc::clone(ws),
                     notify_tx,
@@ -762,6 +762,7 @@ impl TestRigBuilder {
             sandbox_readiness: ironclaw::agent::SandboxReadiness::Available, // tests don't use real Docker
             builder: None,
             llm_backend: "nearai".to_string(),
+            tenant_rates: std::sync::Arc::new(ironclaw::tenant::TenantRateRegistry::new(4, 3)),
         };
 
         // 7. Create TestChannel and ChannelManager.