Triage bot and PR-based claims (#2397)

* Claim bot: a PR referencing an issue claims it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TPWBBSRPfPYWgrrhbznewR

* Triage bot: label new issues and PRs, /triage, backfill

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TPWBBSRPfPYWgrrhbznewR

* Docs: a draft PR is the claim; labels and triage

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TPWBBSRPfPYWgrrhbznewR

* Triage bot: start a runner only for events it acts on

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TPWBBSRPfPYWgrrhbznewR

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Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -129,7 +129,8 @@ People trust SolidCraft with their designs; a crash loses their work. **This out
 - Before working on issue #N, check nobody has it: `gh pr list -R bherbruck/solidcraft --state open --search "#N"`.
 - If none, branch from the latest `origin/main`, `git commit --allow-empty -m "claim: #N"`, push,
   and open a **draft** PR titled `<summary> (#N)` right away. **The lowest-numbered open PR for an
-  issue wins**; if yours lost, close it. Don't claim by commenting or by editing files.
+  issue wins**; if yours lost, close it. Opening the PR is the claim: the bot assigns you and
+  labels the issue `in progress` (or says on your PR who holds it). Don't claim by editing files.
 - Push work in progress at least daily. A draft with no pushes for 72 hours may be taken over:
   comment on it first, then open your own claim (you may start from its branch).
 - On a shared build machine, check the load before a big build (`cut -d' ' -f1 /proc/loadavg`)