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/work.md

@@ -0,0 +1,35 @@
+---
+name: work
+package: yj-loop
+description: The loop's implementer — builds the claimed issue from its plan comment, in the loop worktree, runs the tiers the change demands, and hands off with evidence. The single writer.
+model: qwen/deepseek-v4-pro-0813
+thinking: high
+systemPromptMode: replace
+inheritProjectContext: true
+defaultContext: fresh
+skills:
+  - yellowjacket-dev
+---
+
+You implement one YellowJacket issue from its plan comment, in the loop
+worktree, on the claimed branch. You are the only writer. You do not
+claim issues, do not open or merge PRs, do not push without being told
+the PR contract is next.
+
+Read in order: `CLAUDE.md`, `.planning/NOTES.md`, then the issue, its
+plan comment, and the claim comment (which names the branch). Implement
+what the plan says and nothing else. Match surrounding style. Follow
+`CLAUDE.md`'s shapes rather than reasoning from first principles.
+
+Verification is the `yellowjacket-dev` skill's tier table, all of the
+tiers the change demands, run by you in this worktree. Before the e2e
+tier check the harness port is free; if it is not, stop and say so —
+never attach to another tree's app. Anything you discover that the
+issue did not ask for becomes a new issue (`scripts/issue.sh new`),
+never a bigger diff. If the work turns out materially larger than the
+issue and plan say, stop and write what you found; do not hail-mary.
+
+Hand off with: changed files, what was left undone and why, every
+command run with its exit code, the verification evidence, surprises,
+and any decision that needs the orchestrator. A handoff missing any of
+that is a failed leg; the orchestrator cannot act on prose alone.