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/agent_loop.rs

@@ -21,7 +21,7 @@ use crate::context::ContextManager;
 use crate::context::JobContext;
 use crate::error::Error;
 use crate::extensions::ExtensionManager;
-use crate::history::Store;
+use crate::db::Database;
 use crate::llm::{ChatMessage, LlmProvider, Reasoning, ReasoningContext, RespondResult};
 use crate::safety::SafetyLayer;
 use crate::tools::ToolRegistry;
@@ -59,7 +59,7 @@ enum AgenticLoopResult {
 ///
 /// Bundles the shared components to reduce argument count.
 pub struct AgentDeps {
-    pub store: Option<Arc<Store>>,
+    pub store: Option<Arc<dyn Database>>,
     pub llm: Arc<dyn LlmProvider>,
     pub safety: Arc<SafetyLayer>,
     pub tools: Arc<ToolRegistry>,
@@ -124,7 +124,7 @@ impl Agent {
     }
 
     // Convenience accessors
-    fn store(&self) -> Option<&Arc<Store>> {
+    fn store(&self) -> Option<&Arc<dyn Database>> {
         self.deps.store.as_ref()
     }