fix(contributor): automatically prepare checkout after grabbing work item

[castrojo]

Problem

The hosted contributor flow can successfully select and /claim a work item, but the contributor workspace remains /home/dev with no checkout of the assigned repository. The agent then reports that it is ready but cannot begin implementation until a human makes the task repository available.

This breaks the expected automatic loop: grab work item → prepare workspace → inspect repo → start work.

Observed behavior

For an assigned projectbluefin/bluefin-lts issue:

• The issue was selected from the GitHub queue.
• /claim was posted successfully.
• The working directory was /home/dev.
• No bluefin-lts checkout existed.
• The agent stopped to ask the user to clone or provide the repository.

Expected behavior

After claiming a work item, the contributor runner should automatically:

1. Resolve the repository named by the work item.
2. Clone or reuse a checkout in the configured workspace.
3. Check out the appropriate base/working branch according to repository policy.
4. Read the repository's AGENTS.md and relevant skill index.
5. Begin repository inspection and implementation without requiring a human handoff.

If preparation fails, the runner should report the concrete failure and retain the claimed task, rather than presenting the task as ready while doing no work.

Acceptance criteria

• A claimed work item results in a usable repository checkout before the agent's first implementation turn.
• Existing checkouts are safely reused or refreshed without destroying unrelated work.
• Repository routing comes from the verified work item, not a hardcoded primary repository.
• Missing permissions, clone failures, or ambiguous routing produce an actionable error.
• The contributor status/progress output makes workspace preparation visible.

[castrojo]

Additional evidence from the first assigned task:

• The runner had successfully selected and claimed projectbluefin/bluefin-lts#391.
• It did not prepare a checkout; /home/dev contained only agent-environment files.
• I had to manually clone projectbluefin/bluefin-lts into /home/dev/src/bluefin-lts before repository work could begin.
• The handoff was therefore not merely unclear status output; workspace preparation was absent from the automatic contributor path.

[castrojo]

Follow-up: the current contributor environment also lacks bats, systemd-analyze, shellcheck, pre-commit, just, and podman. The common unit-test workflow installs bats but does not validate systemd unit ordering with systemd-analyze. Please assess whether the contributor image should provide a small validation toolset or expose a documented disposable validation container; at minimum, task startup should make missing validation tools visible.