feat(acp): session history — list and resume past conversations

[bug-ops]

Problem

ACP clients (e.g. Zed) display a "Recent" panel listing previous sessions and allow users to resume them. Zeph currently stores ACP session events in SQLite (acp_sessions + acp_session_events tables via SqliteStore) but exposes no API surface to list or resume past sessions. Clients have no way to query history and render the panel.

Solution

1. ACP transport — new endpoints

Implement two new HTTP/WS endpoints in zeph-acp transport layer:

• GET /sessions — returns a paginated list of sessions with metadata:

  [
    { "id": "...", "title": "...", "created_at": "...", "updated_at": "...", "message_count": 5 }
  ]

• GET /sessions/{session_id}/messages — returns the full event log for a session so the client can replay it.

2. ACP agent — resume protocol

When a client sends POST /runs (or opens a WS) with an existing session_id, the agent must:

1. Load stored events from SqliteStore::load_acp_events.
2. Reconstruct conversation context and inject it into the ContextBuilder before the first LLM call.
3. Continue appending new events to the same session row.

Currently SqliteStore::create_acp_session uses INSERT OR IGNORE, so re-using an existing ID is safe — but the agent loop does not yet load prior context on resume.

3. Session title inference

After the first assistant turn, auto-generate a short title (first ~60 chars of the first user message, or a one-shot summarisation call) and persist it via SqliteStore::update_session_title.

4. TUI — session browser panel

Add a collapsible session history panel (keybind: H) to zeph-tui:

• List recent sessions with truncated title + relative timestamp.
• Enter switches the active session, loading its context.
• D deletes a session (with confirmation prompt).

5. Config

Add [memory.sessions] section to config.toml:

[memory.sessions]
max_history = 100          # sessions to keep; 0 = unlimited
title_max_chars = 60

6. CLI

zeph sessions list              # print sessions table
zeph sessions resume <id>       # open existing session in interactive mode
zeph sessions delete <id>       # remove session and events

Affected crates

Crate	Change
zeph-acp	new HTTP handlers, resume logic in agent loop
zeph-memory	list_acp_sessions already exists; add updated_at + message_count columns
zeph-tui	new session browser panel
zeph-core / src/main.rs	CLI subcommands, config struct

Acceptance criteria

• GET /sessions returns sessions ordered by updated_at DESC.
• Client sending an existing session_id receives conversation context from previous turns.
• Session title is auto-populated after the first assistant reply.
• TUI shows recent sessions and allows switching.
• zeph sessions list prints a table with ID, title, date.
• Unit tests cover resume context reconstruction and title inference.

References

• Zed ACP panel screenshot: shows "Recent" list with timestamps and "View All" shortcut.
• Existing storage: crates/zeph-memory/src/sqlite/acp_sessions.rs.
• ACP spec: https://agentclientprotocol.com/get-started/introduction