feat(helexa-acp): session/list so Zed can discover sessions to resume

Stage 3b only implemented the trailing half of resume: write
sessions to disk + handle session/load. But Zed (and any ACP
client) needs `session/list` to discover *which* session belongs
to the workspace it's reopening — without it, the client only
knows how to mint new sessions and resume never fires even
though the JSON sits ready on disk.

Add the missing pieces:

- store::list / list_in_dir — enumerate {id}.json under
  sessions_dir(), optionally filter by cwd, sort recent-first.
  Skips unparseable files with a warn rather than aborting.
- store::unix_to_iso8601 — RFC 3339 formatter for
  SessionInfo.updated_at; pulls chrono in directly (already in
  the dep tree transitively).
- agent::handle_list_sessions — wires the request to the store,
  builds SessionInfo entries with derived titles (first user
  turn, truncated to 60 chars).
- agent::initialize_response — advertise
  session_capabilities.list = {} alongside the existing
  load_session: true.

Verified end-to-end against the user's real hxa-1.json
(60-turn beat conversation): `session/list` returns the entry
with cwd, derived title, and ISO 8601 timestamp.

4 new store unit tests for list filtering, missing-dir
handling, unparseable-file skipping, and ISO 8601 formatting.

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

crates/helexa-acp/Cargo.toml

@@ -33,6 +33,10 @@ tokio-util = { version = "0.7", features = ["rt"] }
 eventsource-stream = "0.2"
 async-stream = "0.3"
 url = { version = "2", features = ["serde"] }
+# Already transitively pulled via the ACP SDK; declared directly so we
+# can format ISO 8601 timestamps for `SessionInfo.updated_at` in the
+# session/list response.
+chrono = { version = "0.4", default-features = false, features = ["std"] }
 
 [[bin]]
 name = "helexa-acp"