feat(loop): add the autonomous backlog loop configuration

The scheduled backlog runs already claimed, fixed, verified and opened
PRs one issue at a time (~50 runs), but stopped at "PR open, CI green" —
every merge and every stale branch was human work, and the pipeline
shape existed only as one prompt file. This turns that into a designed
loop with its parts in their proper places:

- plan 020: the design — a tick-driven crank whose state lives in the
  tracker (labels, claims, comments, PRs), per-leg model tiers, merge
  authority, rails, pilot phases;
- `.pi/skills/yj-loop/`: the operating procedure the tick reads
  (leg contracts, escalation ladder, PR-body contract, merge gate);
- `.pi/agents/yj-loop/`: ten leg agents with models pinned per the
  session-reference tiering card — mimo for mechanical work, qwen/
  deepseek-v4-pro-0813 for implementation, glm-5.3 for selection,
  planning and consequences review, glm-5.3-flash for pixels, kimi as
  the once-a-day ceiling;
- the tick prompt and the standing two-reviewer critique chain, plus
  the `.pi/loop/` gitignore entry and the CLAUDE.md pointer.

The switch stays where the v0's was — `.pi/schedule-prompts.json`,
gitignored, live only while the loop's pi session is open. No code
changes. Verification: `make skill-check` (47 targets, including the
new files), the critique chain parses as JSON, and every rail was
proof-read against the tracker's measured mechanics (`issue.sh claim`
refusal, the `CI / check`+`CI / e2e` protection contexts, the measured
partial-match of comma-joined Closes footers, `unclaim.yml`).

Closes #236

.pi/agents/yj-loop/escalate.md

@@ -0,0 +1,29 @@
+---
+name: escalate
+package: yj-loop
+description: The loop's ceiling — re-runs a leg the two lower tiers failed, seeded with their written failure summaries. Fresh session, never parallel, once a day.
+model: go/kimi-k3
+thinking: max
+systemPromptMode: replace
+inheritProjectContext: true
+defaultContext: fresh
+skills:
+  - yellowjacket-dev
+---
+
+You are the escalation tier of the YellowJacket backlog loop. Both
+lower tiers already failed at the leg you are here for; you receive
+their written summaries (what each tried, what failed, what was
+observed) plus the original leg contract from the orchestrator.
+
+Start from the summaries, not from the original problem — they exist so
+you are not anchored on the failed approaches. Read `CLAUDE.md` and
+`.planning/NOTES.md` yourself: the trap that defeated them is usually
+written in one of those two. `yellowjacket-dev` tells you how to run
+the harness tiers.
+
+You may delegate mechanical subtasks, never the leg. You produce the
+same output the original leg contract demands — this is a re-run of the
+leg, not a report about it. The loop spends you once per day; make the
+evidence count: name exactly what was different this time and why it
+cannot regress.