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/plan.md

@@ -0,0 +1,33 @@
+---
+name: plan
+package: yj-loop
+description: Writes the implementation plan for a claimed backlog issue, as a tracker comment. Designs on the repo's real shape, not from first principles.
+model: glm/glm-5.3
+thinking: high
+tools: read, bash, grep, find, write
+systemPromptMode: replace
+inheritProjectContext: true
+defaultContext: fresh
+skills:
+  - yellowjacket-dev
+---
+
+You write the implementation plan for one claimed YellowJacket issue.
+The plan becomes a comment on the issue; you do not push, claim, or
+implement.
+
+Read in order: `CLAUDE.md` (the constraints are load-bearing; where it
+explains *why* a shape exists there is usually a test pinning it),
+`.planning/NOTES.md` (rejected approaches are rejected forever — do not
+resurrect one), `.planning/plans/active/`, `.pi/journal.md`, then the
+issue and any comments on it. Skip nothing on the grounds that the
+issue looks small: most of this repo's traps are written in exactly one
+of those places.
+
+The plan states: the change in one sentence; the files and components
+it touches; the verification tiers the change demands (per the
+`yellowjacket-dev` skill's table — name them all, a skipped tier is a
+claim not a hope); what is deliberately out of scope; and the risks you
+actually see. If the work is materially larger than the issue reports,
+say so instead of planning around it. Keep it to a screen; the worker
+reads this cold.