fix(run): stop Start/Resume from silently no-oping on an idle lane session

spawnRun adopted any existing `ccam-lane-<id>` tmux session without looking
at it, so a Resume issued while the session sat at a bare shell prompt (left
by `ccam lanes shell`, or by a `claude` that had already exited) dropped the
whole argv: no `--resume` ran, no initial prompt was typed, and the API still
answered 200. The lane's DB `run_id` is not set in that case, so the ERUNLIVE
guard in the start route never saw it either.

Reuse the pane instead of erroring: when the session exists and
`#{pane_current_command}` is a shell, type the argv into that pane and record
the run. A pane running a program (a live `claude`, an editor, a build) is
still adopted untouched, so attaching shows what is running rather than typing
over it. `sendCommand` POSIX single-quotes every argument and uses
`send-keys -l`, the only place in tmux.js that composes a command line.

docs/LANES.md

@@ -327,6 +327,8 @@ An external session is deduped against a dashboard run with the same `session_id
 
 External rows have **no Attach action**: the dashboard owns no tmux session for them, so there is no PTY to bridge. Their action is **Resume**, which does what resuming from history does — `POST /api/lanes/ensure` for the session's `cwd`, then `POST /api/lanes/:id/start` with `resumeSessionId` — spawning a *new* tmux-backed `claude --resume <session>` in that folder. The original terminal keeps running; resuming gives you a second Claude Code process on the same transcript, not a view of the first one.
 
+**Start and Resume are create-or-reuse, and never silently swallow the request.** When the lane's `ccam-lane-<id>` tmux session does not exist, it is created with the full argv. When it exists but its pane is sitting at a **shell prompt** (a `ccam lanes shell` you opened, or a `claude` that has since exited), the argv is typed into that pane — so `--resume` really runs, and an initial prompt really lands, in the session you are already looking at. Only when the pane is running something (a live `claude`, an editor, a build) is the request adopted as-is: attaching shows you what is running rather than typing over it. Before this, an existing session was always adopted silently, so the first Resume after a `ccam lanes shell` answered `200` while doing nothing at all.
+
 The UI operates on a working directory (`cwd`), not a lane id. Starting a run in a `cwd` that no lane owns calls `POST /api/lanes/ensure` first, to idempotently find or adopt a lane for that path; a `cwd` an existing lane already owns is matched from the loaded lane list without a round trip. Either way the run is then started through `POST /api/lanes/:id/start` rather than directly through `POST /api/run`.
 
 ### Finding or adopting a lane: `POST /api/lanes/ensure`