mark an issue in-progress when the watcher assigns a volunteer to it

[atomantic]

Summary

The issue-watcher already assigned a contributor who commented asking for an issue — its claim regex matches phrasings like "I'd like to work on this — could you assign it to me?" — but it only set the assignee. The issue kept reading as unclaimed in the Issues tab, so the claim was finished by hand every time.

• Stamp in-progress alongside the assignee. One combined gh issue edit is the fast path. Because --add-label fails the whole call on a repo that has never defined the label, the fallback splits it: assign alone (the half that must not be lost on a fork), create the label without --force (so an install that recolored it keeps its color), then apply it.
• A label failure never changes the assignment's result — the comment is still retired rather than re-spending a cognition run. A missing assignment is the failure that matters, and it still routes the comment to the reasoning agent.
• IN_PROGRESS_LABEL moves to lib/dispatchLabels.js beside the other workflow markers, which is what lazily creates them. perpetualWork.js's skip-list now reads that constant instead of repeating the literal, and issueReconcile.js's dead re-export is gone (nothing imported it from there).

This path is fully deterministic — no model is involved in claim detection or assignment.

Known divergence

taskPromptDefaults/prompts.js tells the claim-issue agent, on this same event, to "leave contributor-invitation labels intact, do NOT add in-progress." The watcher now does the opposite, so the two paths disagree. Reconciling the prompt needs a PROMPT_VERSIONS bump plus a migration, which doesn't belong here — filed separately, along with two consequences:

• A volunteer-held issue with no PR yet classifies STALLED on each issue-reconcile pass. That is log-only — no agent dispatch, no spend.
• The watcher does not release good first issue / help wanted the way a claim does.

Test plan

• server/services/issueWatcher.test.js — 5 volunteer tests on a shared fixture, each pinning a distinct regression: the combined edit is issued; the real-world question phrasing still claims; the label is created and re-applied on a repo that lacks it; a label failure keeps the assignment; an assignment failure still routes to cognition.
• server/lib/dispatchLabels.test.js — the new workflow label resolves a spec and a label create command.
• Full server suite: 38,596 passed, 24 skipped.