Resume a persisted ACP session instead of always starting a new one

[tmecklem]

The problem

ACPRuntime.Session is started from a persisted session row, and that row already carries an acp_session_id whenever the session has run before — after the local runtime crashed, or was lost to a host restart.

Client.start_session/7 ignored it and always called session/new. Two consequences:

• The user got a fresh, empty transcript for work that was still there. The agent still held the conversation; nothing asked for it back.
• The previous session was stranded inside the agent, with nothing pointing at it.

Downstream (in Launchbox) this shows up as a coding-agent tab that silently disappears when you close and reopen a card: the runtime can be restarted from the row, but reconnecting produced a blank session, so there was nothing worth showing.

The fix

start_session/7 accepts a :resume_session_id, which Session.connect/2 fills in from the persisted acp_session_id. When one is present the client calls session/load, and the agent replays the conversation as session/update notifications.

opts = Keyword.put(state.client_options, :resume_session_id, state.acp_session_id)

The load is best-effort. An agent that doesn't advertise loadSession, or one that has since expired the session, falls back to session/new rather than failing the connect — a resume that can't be honoured still has to leave the user with a working agent.

Behaviour is unchanged when there's nothing to resume (acp_session_id is nil), which is every session that has not run before.

Notes

• ExMCP.ACP.Client.load_session/4 already existed and gates on the agent's loadSession capability internally, so there's no duplicated capability check here — an unsupported load simply returns an error and falls through.
• No public API breakage: start_session/7 keeps its arity and the new value rides in the existing opts keyword, so alternative client_module implementations are unaffected.

Test plan

mix precommit — 32 tests, clean build, formatted.

New coverage:

• client_test.exs — a resumed session sends session/load with the right sessionId/cwd, and returns that id.
• client_test.exs — an agent that rejects the load falls back to session/new and still returns a working session.
• session_test.exs — a persisted acp_session_id is passed down as :resume_session_id; a session with nothing to resume passes nil.

Also verified against a real consumer: pointed Launchbox at this checkout as a path dep and confirmed a Launchbox-persisted acp_session_id reaches the client as :resume_session_id, and that the assertion fails without the Session.connect/2 change. Launchbox's own suite passes against this branch.