feat(skill): adr — the house format, and how to record a decision you cannot make yet

Codifies cad's ADR template (frontmatter + Status/Context/Decision/Consequences/
Validation) and formalises the productive-tensions pattern that ADR-0001 already
does informally under "Known refinements, deliberately not built yet".

The RED run is why the skill is shaped this way. An unaided agent given a
genuinely-undecidable transport choice produced a strong ADR — it invented a
"Cost of reversal" section, wrote concrete revisit triggers, and reframed the
unanswerable question ("which is better?") into an answerable one ("which
option's correctness depends on the unsettled variables?"). So the reasoning was
not the gap. What it could not guess was the house format: it invented a bullet
header instead of the frontmatter that makes the corpus greppable, and it wrote
no Validation section at all.

The GREEN run then found eleven problems. Four mattered:

- **The Overview contradicted the Escalation section in its first sentence** —
  "An ADR is not documentation of a decision. It is the decision" against
  "decides nothing". Escalation is right; the flourish is gone.
- **The escalation clause trapped every agent-authored ADR.** The deciders are
  never the agent, so `proposed` is mandatory — and the skill separately called
  a lingering `proposed` a failure mode. Now: a `proposed` ADR with a named
  decider is waiting on a person and gets chased; one without is waiting on
  nobody. Naming that person is the deliverable, not a formality.
- **No filename or path convention, in a skill whose thesis is greppability.**
  Called "the biggest hole in the skill", correctly. Now docs/adr/NNNN-slug.md,
  with the cross-repo caveat that a bare number is ambiguous between repos.
- **"Pick the option whose correctness does not depend on the unknown" assumed
  such an option always exists.** It often does not — that is what makes a
  tension productive — and the rule would then push the author to manufacture a
  reason one of them is, which reads as reasoned and never gets re-examined. A
  fallback ladder now says: cheaper to reverse, lower switching cost, already
  built — and say that is why.

Also: `deciders` had no valid value while proposed; the body Status section
duplicated frontmatter with no precedence; the tension template silently dropped
the "address the strongest case against" requirement the plain format demands;
the preservation strategy — the actual technical content — got one angle bracket
while the easy parts got full templates; resolution triggers were singular for
tensions that routinely have several; and lineage/grill-me were listed as
cross-references at the bottom when they are pre-steps that change the content.

Closes #9
Bump-Type: minor

SKILLS_INDEX.md

@@ -9,6 +9,7 @@ the SKILL.md, not the table.
 <!-- BEGIN skills-table -->
 | Skill | Description | Use when | Surfaces |
 |-------|-------------|----------|----------|
+| `adr` | Architecture Decision Records — house format, and how to record a decision you cannot make yet | Recording an architectural decision; revisiting one; or deciding between options where the right answer genuinely is not knowable yet | claude-code, library |
 | `atdd` | Acceptance Test-Driven Development workflow | Feature development with acceptance criteria | claude-code, library |
 | `cad-babysit` | Focused human-in-the-loop session working CAD's stuck queue (read-only) | Picking up whatever `cad stuck` flags, with a human present | claude-code, library |
 | `clean-code` | Clean code principles with Go adaptation | Code review, refactoring, new code | claude-code, library |
@@ -21,6 +22,7 @@ the SKILL.md, not the table.
 | `feature-spec` | Implementation-level feature spec (tighter than spec-driven-dev) | Scoping one feature inside an already-specced project | claude-code, library |
 | `gitea-ci` | Gitea CI/CD pipeline with act_runner, buildah, k3s, GitHub mirror | Setting up or debugging CI on this infra | claude-code, library |
 | `grill-me` | Structured plan interrogation — Quick Poke, Full Grill, Pre-mortem | Stress-testing a plan before committing; end of Diamond 1; before promoting to pre-prod | claude-code, library, chat |
+| `lineage` | Backward decision archaeology — was this tried before, and does the reason it failed still hold | Before proposing something that feels new; when an idea keeps resurfacing; when you inherit code whose reason is gone | claude-code, library |
 | `own-the-task` | Delegation ownership — Intake → Brief → Plan → Execute | A task is handed over open-ended; \"take this off my plate\"; before a plan exists | claude-code, library, chat |
 | `planning` | Planning and task breakdown | Sprint planning, task decomposition | claude-code, library |
 | `playwright` | Playwright E2E testing — setup, HTMX patterns, SSE streams, CI wiring | Adding browser tests to any project with a web UI | claude-code, library |