Fire failure-lesson capture on root-cause-confirmed + add supersedes (close #664/r13 gap)

Overview / Motivation

The epm:failure-lesson capture step fires only when "a crash-fix round RESOLVES the failure" (round resolved + relaunch succeeded). This missed the #664 r13 root cause: the real cause of the multi-relaunch "vLLM hang" was per-call AutoTokenizer.from_pretrained reloads inside _render() (fixed with lru_cache), found in r13 — but r13 immediately hit the next p2 bug, so the "this round resolved X, here's the lesson" capture never fired. Result: the WRONG intermediate lessons (chunk-it / vLLM-EngineCore-deadlock) were captured durably into gotchas.md, and the CORRECT root cause is in no durable file. Worse, there is no retraction path: when an early lesson is a mis-diagnosis, nothing supersedes it.

Goal

(1) Fire failure-lesson capture on ROOT-CAUSE-CONFIRMED, not only on round-resolved-and-relaunch-succeeded. (2) Add a supersedes: field so a corrected diagnosis retracts/annotates the earlier wrong lesson in the durable record.

Scope / surfaces

• .claude/skills/issue/SKILL.md — the "Failure-lesson capture" step: add the root-cause-confirmed trigger (a round that identifies the true cause posts the lesson even if a subsequent, distinct failure follows) and the supersedes: <prior-lesson-slug|marker-ts> field.
• .claude/agents/experiment-implementer.md + .claude/agents/experimenter.md — the failure-lesson block spec: add supersedes: and the root-cause-confirmed firing condition.
• .claude/workflow.yaml — the epm:failure-lesson marker schema: add supersedes (optional).
• Consumer side: when a lesson carries supersedes:, the agent-memory write / gotcha promotion must update-or-annotate the superseded entry, not just append (coordinate with the deterministic consolidator task).

Constraints / invariants

• Workflow-surface only.
• Backward compatible: lessons without supersedes: behave exactly as today.
• scripts/workflow_lint.py --check-asks + ruff pass; if workflow.yaml or CLAUDE.md change, keep them consistent with the rule/agent files.

Acceptance

• A two-round fixture (round A posts a mis-diagnosis lesson; round B posts the corrected lesson with supersedes:) results in the durable record carrying the corrected cause and the superseded one marked/annotated, not both as co-equal gotchas.
• A root cause confirmed in a round that then hits a NEW distinct failure still produces a captured lesson.

Provenance note

Surfaced during a PM-session audit of #664's vLLM bug-bounce: the r13 tokenizer-reload root cause was never durably captured while the chunk-it mis-diagnosis was promoted to gotchas.md. Related (deferred): the #664-specific gotchas.md correction, held on predicate-664-lands.