docs: triage Qwen worker budget failures (#371)

[bearmug]

Summary

• document the observed Qwen worker failure as a pi-subagents tool-budget outcome, not a turns quota or current llama.cpp health failure
• add health/model/smoke-test triage commands and bounded-worker operating guidance
• link the runbook from the README

Closes #371

Validation

• git diff --check ✅
• local Qwen /health ✅
• exact /v1/models model and 131072 context ✅
• chat smoke test returned OK ✅
• npm run check ⚠️ blocked in the fresh worktree because dependencies are not installed (tsc: command not found)
• fresh reviewer: no blockers/majors; docs-only change
• codex review --base origin/main: no actionable findings

Scope

This does not change llama.cpp settings, runner budgets, fallback behavior, or server lifecycle. The machine-readable resumable outcome remains an upstream pi-subagents follow-up tracked by this issue.

[bearmug]

Pre-merge validation reconciliation: commit 57eed67 had a CI test failure in the repository binary-dataset gate (Binary dataset is weak-label research only (0/2 reviewed)), unrelated to this documentation-only change. A transient gh pr checks query also returned no checks while the replacement run was queued. After the final evidence correction at commit 1fd2046, CI run 29545289289 completed successfully: type/validation coverage, tests, and packed minimum-runtime durability all passed. Current PR head is 1fd2046. No merge performed.

[bearmug]

Feature-loop completion record for Issue #371: investigation completed from the Qwen child session and runner metadata; diagnosis documented as a 600-second runner timeout after soft tool-budget pressure (turn quota and hard tool limit ruled out); resolution artifacts are in docs/qwen-worker-failure-triage.md and docs/evidence/qwen-worker-tool-budget-2026-07-17.json; pre-merge validation completed successfully on head 1fd2046 (run 29545289289, attempt 2). Issue: #371. PR: #373. No merge performed.

[bearmug]

Final validation reconciliation: valid gh pr checks 373 returned check (22.19.0) pass for head 1fd2046. Valid gh run view 29545289289 --json headSha,status,conclusion,attempt,jobs returned head 1fd2046, attempt 2, conclusion success, and job check (22.19.0) completed successfully. The earlier failed validation was a CLI read/query error using unsupported runAttempt, not a failed PR check. PR remains open and mergeable; no merge performed.