feat(helexa-acp): session resume via session/load

Zed restarts (frequent during helexa-acp dogfooding) used to lose
every conversation because we'd ignore the load_session capability
and treat every project-reopen as a fresh session/new. Persist
sessions to disk and honour session/load so the agent panel comes
back where it left off.

Storage layout:
  $XDG_DATA_HOME/helexa-acp/sessions/{session_id}.json

Each file holds session_id, cwd, model_id, mode_id, full Message
history, plus created/updated timestamps. Atomic save via
tempfile+rename so a crash mid-write can't corrupt the store.

Touch points:

- src/store.rs (new) — sessions_dir() resolution, save/load via
  default and explicit-dir entry points (so unit tests don't have
  to race on XDG_DATA_HOME). 5 unit tests cover round-trip,
  not-found errors, atomic overwrite, tool-call/result preservation,
  and the filename sanitiser's path-traversal handling.
- src/provider/mod.rs — Serialize/Deserialize on Role, Message,
  MessageContent, ToolCall. MessageContent::Text turned into a
  struct variant ({text: ...}) so internally-tagged JSON works.
- src/agent.rs — initialize_response advertises load_session: true;
  handle_load_session reads the file, snapshots in-memory state,
  returns LoadSessionResponse with the persisted mode preselected;
  drive_prompt persists at the end of every prompt round under the
  session lock with the I/O outside the lock.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/helexa-acp/src/session.rs

@@ -152,7 +152,9 @@ mod tests {
             .history
             .push(Message {
                 role: Role::User,
-                content: MessageContent::Text("hello".into()),
+                content: MessageContent::Text {
+                    text: "hello".into(),
+                },
             });
 
         assert_eq!(