fix(agent): persist /model selection to .env, TOML, and DB (#1581)

* fix(agent): persist /model selection to .env, TOML, and DB

The /model command only wrote selected_model to the DB and config.toml,
but env vars from ~/.ironclaw/.env (e.g. NEARAI_MODEL) have the highest
priority in LlmConfig::resolve_model(). The .env value was never
updated, so it always shadowed the new model on restart.

Now persist_selected_model updates all three persistence layers:
1. The backend-specific model env var in ~/.ironclaw/.env (only if the
   var already exists, to avoid injecting new vars)
2. The config.toml file (created if absent, since TOML > DB priority)
3. The DB settings table (for completeness)

Also adds diagnostic logging when the DB store is unavailable.

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

* fix(agent): address PR review — backend from deps, exact .env match

Review feedback:
- Use resolved llm_backend from AgentDeps instead of re-reading from
  disk/env (fixes DB-only backend detection, eliminates redundant I/O)
- Match .env var with exact "KEY=" prefix and skip commented lines
  (prevents false matches on NEARAI_MODEL_VERSION etc.)
- TOML is now loaded once (no double-read for backend + model update)

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

---------

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

tests/e2e_telegram_message_routing.rs

@@ -200,6 +200,7 @@ mod tests {
             document_extraction: None,
             sandbox_readiness: ironclaw::agent::SandboxReadiness::DisabledByConfig,
             builder: None,
+            llm_backend: "nearai".to_string(),
         };
 
         let gateway = Arc::new(TestChannel::new());

tests/support/gateway_workflow_harness.rs

@@ -264,6 +264,7 @@ impl GatewayWorkflowHarness {
                 document_extraction: None,
                 sandbox_readiness: ironclaw::agent::SandboxReadiness::DisabledByConfig,
                 builder: None,
+                llm_backend: "nearai".to_string(),
             },
             channels,
             None,

tests/support/test_rig.rs

@@ -761,6 +761,7 @@ impl TestRigBuilder {
             document_extraction: None,
             sandbox_readiness: ironclaw::agent::SandboxReadiness::Available, // tests don't use real Docker
             builder: None,
+            llm_backend: "nearai".to_string(),
         };
 
         // 7. Create TestChannel and ChannelManager.