Add workspace and memory system (OpenClaw-inspired)

Implements persistent memory for agents with hybrid search:

- Database-backed workspace with PostgreSQL (not filesystem)
- Memory documents: MEMORY.md, daily logs, identity files
- Chunked content with FTS (tsvector) + vector (pgvector) indexes
- Reciprocal Rank Fusion (RRF) for hybrid search combining BM25 and semantic
- Memory tools: memory_search, memory_write, memory_read
- Proactive heartbeat system for periodic execution (30 min default)
- OpenAI embeddings provider (text-embedding-3-small)

Key patterns from OpenClaw:
- "Memory is files, not RAM" - explicit persistence required
- Two-tier memory: daily logs (raw) + curated MEMORY.md
- Session isolation via user_id/agent_id scoping

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/error.rs

@@ -36,6 +36,9 @@ pub enum Error {
 
     #[error("Repair error: {0}")]
     Repair(#[from] RepairError),
+
+    #[error("Workspace error: {0}")]
+    Workspace(#[from] WorkspaceError),
 }
 
 /// Configuration-related errors.
@@ -268,5 +271,30 @@ pub enum RepairError {
     },
 }
 
+/// Workspace/memory errors.
+#[derive(Debug, thiserror::Error)]
+pub enum WorkspaceError {
+    #[error("Document not found: {doc_type} for user {user_id}")]
+    DocumentNotFound { doc_type: String, user_id: String },
+
+    #[error("Search failed: {reason}")]
+    SearchFailed { reason: String },
+
+    #[error("Embedding generation failed: {reason}")]
+    EmbeddingFailed { reason: String },
+
+    #[error("Document chunking failed: {reason}")]
+    ChunkingFailed { reason: String },
+
+    #[error("Invalid document type: {doc_type}")]
+    InvalidDocType { doc_type: String },
+
+    #[error("Workspace not initialized for user {user_id}")]
+    NotInitialized { user_id: String },
+
+    #[error("Heartbeat error: {reason}")]
+    HeartbeatError { reason: String },
+}
+
 /// Result type alias for the agent.
 pub type Result<T> = std::result::Result<T, Error>;