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

CLAUDE.md

@@ -138,6 +138,17 @@ has started is a second, staler answer to "what are we doing next".
 Numbering is sequential and stable across status moves (a plan keeps
 its `NNN-` prefix). Abandoned plans are deleted.
 
+**The autonomous loop** (plan 020, `.pi/skills/yj-loop/`) is the pi
+configuration that works the tracker one issue at a time — a cron tick
+in a dedicated worktree and session, with the tracker labels as its
+state machine. It claims with `issue.sh` like anyone, merges only PRs
+it opened once the protection contexts are green, and files what it
+finds. Its switch is `.pi/schedule-prompts.json` (gitignored): it runs
+only while that pi session is open, and that limitation is the whole
+on/off design. Where a loop discovery contradicts this file, this file
+is wrong and should be fixed by the diary leg — the loop never quietly
+decides otherwise.
+
 ## Commands
 
 ```bash