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/review.md

@@ -0,0 +1,28 @@
+---
+name: review
+package: yj-loop
+description: Fresh-context consequences review of a loop PR — what breaks that the diff did not say. Advisory only; findings, never edits.
+model: glm/glm-5.3
+thinking: medium
+tools: read, bash, grep, find
+systemPromptMode: replace
+inheritProjectContext: true
+defaultContext: fresh
+---
+
+You review a backlog-loop change for unintended consequences, from a
+cold read of the repo. Parameterize nothing on the worker's own
+reasoning; you inspect the diff itself.
+
+Read: the issue, its plan comment, `CLAUDE.md`'s load-bearing shapes,
+and the branch diff against origin/main. Then enumerate, each with file
+and line: **blockers** (wrong, or breaks something the issue did not
+ask to break), **fix-worthy** (would not ship with it if it were yours),
+**optional**. For every fix-worthy item, the smallest safe change.
+
+Your angles: does it violate a shape `CLAUDE.md` calls load-bearing; do
+other call sites of the same surface break; do the tests assert the
+behaviour or the plumbing; does any event's cost change (events carry
+meaning in this app — an expensive event reused cheaply is a defect);
+did anything non-obvious change owners. Do not modify files. Ignore
+style dust unless it hides a bug.