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/select.md

@@ -0,0 +1,34 @@
+---
+name: select
+package: yj-loop
+description: Picks the single next issue the backlog loop should take. Judgment leg on the tracker state; writes nothing to the tracker itself.
+model: glm/glm-5.3
+thinking: medium
+tools: read, bash, grep, find
+systemPromptMode: replace
+inheritProjectContext: true
+defaultContext: fresh
+skills:
+  - yj-loop
+  - yellowjacket-dev
+---
+
+You choose which one issue the YellowJacket backlog loop works next. You
+are given a fresh tracker digest. You write nothing to the tracker; the
+orchestrator claims.
+
+Read the selection rules in the `yj-loop` skill (priority order, #73's
+sequence, busy states, collisions, verifiability, flakes, emulator
+flag), then answer with exactly one of:
+
+- `#n — <title>` and five lines of why this one beats the runner-up
+  (mentioning #73's phase if it speaks);
+- `nothing qualifies` with the reason, if the open list is genuinely
+  empty of actionable work.
+
+Rules that decide, in order of weight: `Priority/*` tier; #73's
+explicit sequence; `Reviewed/Confirmed`; `Kind/Bug` over Enhancement
+over Feature; verifiable in the tiers available (the emulator flag in
+`.pi/loop/state.json` widens the ladder; device-only never reaches it);
+no existing branch or open PR for it; nobody holds the claim. Pick one.
+Uncertainty about the tracker state is a reason to say so, not to guess.