Add session persistence and Cmd+K command palette

Workspace auto-saves every 30s and on exit, restores all tabs on
launch. Sessions persist drawing elements, SVG source, canvas state,
and metadata to ~/Library/Application Support/agcanvas/workspace.json.
Manual save via Cmd+S.

Command palette (Cmd+K) with fuzzy search over all commands: session
management, tool switching, view toggles, canvas operations. Arrow
keys to navigate, Enter to execute, Esc to dismiss.

README.md

@@ -35,6 +35,8 @@ agcanvas bridges the gap between visual design and code generation. It's a **col
 - **Mermaid Diagrams** — Write Mermaid syntax, render as SVG on canvas
 - **Sessions/Tabs** — Multiple canvases in tabs, each with independent state, creator tracking (human vs agent), descriptions, and timestamps
 - **Pan/Zoom** — Smooth canvas navigation
+- **Session Persistence** — Auto-saves workspace to `~/Library/Application Support/agcanvas/`, restores all tabs on launch
+- **Command Palette** — Cmd+K to search and execute any command with fuzzy matching
 
 ### AI Agent Integration
 - **MCP Server** — `agcanvas-mcp` bridge for Claude Code, OpenCode, and Codex
@@ -132,6 +134,8 @@ agcanvas-mcp --help
 | Paste SVG | Cmd+V |
 | New Tab | Cmd+T |
 | Close Tab | Cmd+W |
+| Save workspace | Cmd+S |
+| Command palette | Cmd+K |
 | Reset zoom | Cmd+0 |
 
 ## MCP Server (AI Agent Integration)
@@ -338,6 +342,8 @@ crates/
 │       ├── main.rs           # Entry point, window setup
 │       ├── app.rs            # Main app state, UI, toolbar, drawing interaction
 │       ├── session.rs        # Session/tab state management
+│       ├── persistence.rs    # Workspace save/load (~/.agcanvas/)
+│       ├── command_palette.rs # Cmd+K command palette with fuzzy search
 │       ├── element_tree.rs   # Structured element representation
 │       ├── clipboard.rs      # System clipboard integration
 │       ├── mermaid.rs        # Mermaid → SVG rendering
@@ -372,6 +378,7 @@ crates/
 | `arboard` | Clipboard access |
 | `tokio-tungstenite` | WebSocket (both server and client) |
 | `rmcp` | MCP server SDK (Anthropic official) |
+| `dirs` | Platform data directory paths |
 | `serde`/`serde_json` | Serialization |
 
 ## Roadmap
@@ -383,6 +390,8 @@ crates/
 - [x] MCP server bridge for AI coding tools
 - [x] Agent draw commands (modify canvas from agent)
 - [x] Session metadata (creator tracking, descriptions, timestamps, sorting)
+- [x] Session persistence (auto-save/restore workspace)
+- [x] Command palette (Cmd+K)
 - [ ] Real code generation (not just stubs)
 - [ ] Export to file
 - [ ] Diff view (before/after agent changes)