swarm: agents-tab detection requires running inside a herdr pane

thrawn swarm finds/creates the shared agents tab by asking herdr pane current for the workspace id. That only works when thrawn is invoked from inside a herdr pane.

Current behaviour when detection fails: a warning is printed and panes fall back to a per-run ⚔ swarm tab (or detached background processes when herdr isn't running at all). Agents still run either way.

Possible improvements:

• Fall back to the focused workspace (herdr workspace API) when there's no current pane, so swarm launched from a plain terminal still lands in the right place
• Config option to pin swarm to a specific workspace

From the swarm mode PR (#4).

🤖 Generated with Claude Code

Agent brief

Safe to land: this improves a fallback path only. When herdr pane current fails because thrawn was not invoked from inside a herdr pane, try resolving the workspace another way before falling back (for example herdr api by matching the repo cwd against workspace paths, if the socket API supports it; read herdr --help and the api subcommands to find out). Keep the existing warning and per-run tab fallback when nothing resolves. Add a test if the resolution logic is a pure function. Acceptance: make test and make lint green; behaviour inside a pane unchanged.

Start in the thrawn swarm detection path that calls herdr pane current, then read herdr --help and its API subcommands to determine whether the focused workspace can be resolved from the repository cwd. Preserve the existing warning and per-run ⚔ swarm fallback when resolution fails, and verify that in-pane behavior is unchanged. Run make test and make lint.