Load session list dynamically from ACP backend

[gitricko]

Summary

The extension loses the session list on browser reload because SessionStore stores everything in VS Code workspaceState — which is in-memory only in code-server. The Hermes ACP backend already has a list_sessions RPC returning all sessions from its own state.db, but the extension never called it.

Fix: On ACP connection, call list_sessions and populate the session list dynamically. Backend is the source of truth — no file fallback, no dual persistence needed.

Changes (4 files, +52 lines)

File	Change
sessionManager.ts	Added listSessions() — calls ACP list_sessions RPC
sessionStore.ts	Added addBackendSession() — inserts a backend session without activating it
chatPanel.ts	Added syncSessionsFromBackend() — merges backend sessions into the store, deduped by acpSessionId
extension.ts	Calls panel.syncSessionsFromBackend() after client.start() succeeds

How it works

Extension activate → client.start() → ACP connected
  → syncSessionsFromBackend()
    → acpClient.call(list_sessions, {})
    → Backend returns [{sessionId, title, updatedAt}, ...]
    → Store adds sessions not already present
    → Session list broadcast to webview

User clicks an old session → switchSession → session/load restores Hermes context → works immediately.

How to test

1. Open the Hermes chat panel and start a few sessions (each creates a row in Hermes state.db)
2. Reload the browser tab
3. Before fix: session list is empty, only "new session" shows
4. After fix: old sessions appear in the list with their titles, clickable and resumable

Tradeoffs

• Old sessions restored from the backend show with their titles but empty message history in the chat panel (messages are in Hermes state.db but not returned by session/load). The session context IS restored on Hermes side — you can continue chatting from where you left off.
• Adding a session/get_history backend endpoint later would let us load the previous turns into the UI too.