feat: add libSQL/Turso database backend with full feature parity

Introduce a Database trait abstraction (~60 async methods) enabling
compile-time backend selection between PostgreSQL and libSQL/Turso.
Convert all modules from concrete Store to Arc<dyn Database>, add
LibSqlSecretsStore and LibSqlWasmToolStore implementations, wire
libsql stores throughout CLI and main entry points, and make the
setup wizard backend-agnostic.

Key changes:
- src/db/: Database trait, PostgresDatabase adapter, LibSqlBackend
  with native SQLite-dialect SQL, and idempotent migration system
- src/secrets/store.rs: LibSqlSecretsStore (all 8 trait methods)
- src/tools/wasm/storage.rs: LibSqlWasmToolStore (all 7 trait methods)
- src/main.rs, cli/tool.rs, cli/mcp.rs: backend-conditional wiring
- src/setup/channels.rs: SecretsContext uses Arc<dyn SecretsStore>
- Feature-gate postgres-only tests and examples

Co-Authored-By: Claude Opus 4.6 <[email protected]>

src/agent/worker.rs

@@ -11,7 +11,7 @@ use crate::agent::scheduler::WorkerMessage;
 use crate::agent::task::TaskOutput;
 use crate::context::{ContextManager, JobState};
 use crate::error::Error;
-use crate::history::Store;
+use crate::db::Database;
 use crate::llm::{
     ActionPlan, ChatMessage, LlmProvider, Reasoning, ReasoningContext, RespondResult, ToolSelection,
 };
@@ -28,7 +28,7 @@ pub struct WorkerDeps {
     pub llm: Arc<dyn LlmProvider>,
     pub safety: Arc<SafetyLayer>,
     pub tools: Arc<ToolRegistry>,
-    pub store: Option<Arc<Store>>,
+    pub store: Option<Arc<dyn Database>>,
     pub timeout: Duration,
     pub use_planning: bool,
 }
@@ -67,7 +67,7 @@ impl Worker {
         &self.deps.tools
     }
 
-    fn store(&self) -> Option<&Arc<Store>> {
+    fn store(&self) -> Option<&Arc<dyn Database>> {
         self.deps.store.as_ref()
     }
 
@@ -382,7 +382,7 @@ Report when the job is complete or if you encounter issues you cannot resolve."#
         tools: Arc<ToolRegistry>,
         context_manager: Arc<ContextManager>,
         safety: Arc<SafetyLayer>,
-        store: Option<Arc<Store>>,
+        store: Option<Arc<dyn Database>>,
         job_id: Uuid,
         tool_name: &str,
         params: &serde_json::Value,