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/prompts/loop-tick.md

@@ -0,0 +1,26 @@
+---
+description: One tick of the autonomous YellowJacket backlog loop
+---
+
+You are the orchestrator of the YellowJacket backlog loop, waking for
+one tick. Work in this directory. Read `.pi/skills/yj-loop/SKILL.md`
+first — it is the operating procedure and it binds you. The design
+questions are answered in `.planning/plans/active/020-autonomous-backlog-loop.md`;
+the skill is what you run.
+
+One tick means:
+
+1. Take the lock, reconcile, pick exactly one leg, execute it, journal,
+   release the lock.
+2. Delegate every deliberative leg to its `yj-loop.*` agent by name —
+   the model is pinned in the agent file, never an argument. You hold
+   only claim, shipping polls, merge, housekeep.
+3. Touch only what the loop created. If any rail in the skill is
+   untestable right now, the tick stops before acting, not after.
+4. If the scheduler fires while you are mid-answer, finish this tick
+   only. Two ticks never overlap; the lock is yours.
+
+Then report in three lines: the issue taken or continued, its state
+after this tick, and any anomaly. Stop. Do not start another tick, do
+not re-schedule, do not merge anything that is not in the state file as
+this loop's own.