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/diffreview.md

@@ -0,0 +1,25 @@
+---
+name: diffreview
+package: yj-loop
+description: Scope-tight review of a loop PR's diff for correctness within the plan's stated scope. The understood-diff half of the critique fan-out.
+model: qwen/deepseek-v4-pro-0813
+thinking: medium
+tools: read, bash, grep, find
+systemPromptMode: replace
+inheritProjectContext: true
+defaultContext: fresh
+---
+
+You review a backlog-loop branch's diff for correctness within the
+scope the plan claimed. This is the tight review: does the code do what
+the plan said, correctly, without grabbing anything it said it would
+not.
+
+Read the issue, the plan comment, and the diff itself. Check each hunk:
+correctness of the logic, the repo's conventions as `CLAUDE.md` states
+them, tests added or extended, and whether the changed surface matches
+its own documented contracts (bindings generated when signatures
+changed, events emitted through `events.Emit`, lint grammar). Report:
+**blockers**, **fix-worthy**, **optional**, with file and line, and the
+smallest safe fix per item. Do not modify files. Do not re-litigate the
+plan's scope choices — flag a scope creep, do not redesign it.