Local job execution via user-installed Pulsar endpoint + relay

Loom should support running jobs locally on the user's machine by routing them through a user-installed Pulsar endpoint connected to the Galaxy relay (`relay.usegalaxy.org`).

## How it works

The Pulsar endpoint:

- Stages inputs over HTTPS from the Galaxy server
- Runs the tool inside Docker on the user's machine
- Returns outputs through the same relay

Working setup on Ubuntu 24.04 (Pulsar in venv, Docker engine, `app.yml` pointing at `relay.usegalaxy.org`, manager named after the user) successfully picks up and executes jobs from `test.galaxyproject.org`. Full setup notes and pain points: galaxyproject/pulsar#452.

## What Loom needs

- Document the Pulsar-endpoint flow as the recommended path for local execution
- Surface endpoint registration and live status in the Loom UI/CLI
- Map a Loom destination to the user's relay manager name

## Blockers tracked upstream (galaxyproject/pulsar#452)

- Single-core default for relay-bound managers
- Shared-password auth in plaintext `app.yml`
- No browser-based endpoint enrollment (the UX target is `gh auth login` / Claude Code style: open browser, approve, store refreshable token)

Loom should not ship local execution to end users until at least the auth story is fixed — handing out plaintext relay passwords does not scale.