A "one-shot" is a single stateless model call that runs OUTSIDE any conversation:
it never touches session history, never breaks prompt caching, and returns plain
text. UI surfaces need this for small generative chores — a commit message from a
diff, a rename suggestion, a summary — where an agent turn would pollute the
thread and hand-rolling an LLM call at every call site would be worse.
- `agent/oneshot.py`: `run_oneshot(...)` over the existing auxiliary-client
plumbing (same path as title generation). Two call shapes: explicit
instructions/input, or a registered `template` + `variables` (templates own the
prompt engineering so it stays consistent across CLI/TUI/desktop). Ships a
`commit_message` template. Model selection inherits the live session via
`main_runtime`, else the configured aux `task` backend.
- `tui_gateway/server.py`: `llm.oneshot` RPC (long-handler) inheriting the
session's model when `session_id` resolves.
Stateless by construction — no session mutation, cache untouched.
