feat: Add support IPC socket control over menu.

docs/DEVPLAN.md

@@ -212,22 +212,68 @@ CSV/TSV input parsing and columnar table rendering.
 **Done when:** `echo "name,age\nalice,30\nbob,25" | pikl --input-format csv`
 renders a navigable table with headers.
 
-## Phase 6: Sessions & IPC
+## Phase 6: IPC & Session History
 
-Persistence and external control.
+Two independent features bundled together. IPC adds live
+external control over a Unix socket. Session history gives
+lightweight filter recall for repeated workflows.
+
+### IPC (External Control)
+
+Opt-in Unix socket server for live control of a running
+pikl instance by external processes.
 
 **Deliverables:**
-- `--session name` for state persistence
-- Session state: filter, scroll position, selections
-- Session history log file
-- Unix socket IPC while running
-- IPC commands: push/remove/update items, set filter,
-  read selection, close
-- Protocol: newline-delimited JSON
+- `--ipc` flag to enable the socket listener (off by
+  default, pikl is ephemeral and shouldn't have side
+  effects unless asked)
+- Socket path: `/run/user/$UID/pikl-{session}.sock`
+  (named session) or `/run/user/$UID/pikl-{pid}.sock`
+  (no session name)
+- Socket path logged via tracing on startup
+- Cleanup on exit (normal, cancel, SIGTERM)
+- Protocol: newline-delimited JSON, one message per line
+- Write commands: `add_items`, `replace_items`,
+  `remove_items`, `set_filter`, `confirm`, `cancel`,
+  `close`, plus navigation actions (`move_up`,
+  `move_down`, `move_to_top`, `move_to_bottom`,
+  `page_up`, `page_down`, `toggle_select`, `select_all`,
+  `clear_selections`)
+- Read commands: `get_state` (filter, cursor, counts,
+  mode), `get_selection` (selected items). Both require
+  an `id` field, response echoes it back.
+- Event subscription: `subscribe` with event type list,
+  `unsubscribe` to stop. Subscribed events pushed as
+  `{"event": "...", ...}` lines.
+- Multiple simultaneous client connections
+- Auth: Unix socket permissions (user-only)
+- IPC is just another frontend: deserializes JSON into
+  Actions, optionally subscribes to MenuEvent broadcast
 
-**Done when:** You can close and reopen a session and find
-your state intact. External scripts can push items into a
-running pikl instance.
+**Not in scope:**
+- `get_items` (full item list read, potentially large)
+- Auto-enable with `--session`
+- Auth beyond Unix socket permissions
+
+### Session Filter History
+
+**Deliverables:**
+- `--session name` flag
+- On any confirm (single, multi, quicklist), append
+  current filter text to
+  `~/.local/state/pikl/sessions/{name}.history`
+- Skip empty filters and consecutive duplicates
+- Load history on startup if session file exists
+- Ctrl+P / Ctrl+N in insert mode to cycle through
+  filter history
+- Selecting a history entry replaces the current filter
+  text
+- Per-session only, no global history
+
+**Done when:** An external script can connect to a running
+pikl instance over the socket, push items, read state, and
+subscribe to events. A named session remembers filter
+history across invocations.
 
 ## Phase 7: Streaming & Watched Sources