fix(skills): plans reference the spec instead of restating it — end to end (SUP-333 A)

Consolidates the reference-discipline change with every consumer of it,
so this PR is independently mergeable (previously split across two
stacked PRs whose intermediate state left the SDD spec reviewer blind).

writing-plans: plans reference the spec — never restate, paraphrase,
or summarize it; spec owns WHAT/WHY, plan owns HOW; cite by path in
the header (**Spec:** template line) and by section where a task needs
context; No Placeholders repetition stays intra-plan; no-spec branch
scoped to conversational-requirements-only (eval-caught: an agent used
an unscoped no-spec branch to skip writing the spec entirely).

brainstorming: spec path loophole closed (claude shortened
docs/superpowers/specs/ to docs/specs/, documented run); an existing
differently-named docs dir is not a "user preference".

subagent-driven-development: Spec Context section — the controller
reads the plan-cited spec and pastes cited sections into implementer
and spec-reviewer prompts; the spec reviewer's diff-only rule gets a
spec-document exception. Without this, reference discipline starves
the pipeline of requirements.

executing-plans: Step 1 reads the spec the plan cites (the
non-subagent path; plans are no longer self-contained).

Eval evidence (quorum, full-stack text): cost-spec-plan-duplication
claude 3/3 pass (RED: 5/5 agents failed), codex pass, pi pass (the
683-line duplication RED agent); sdd-spec-context-consumed functional
pass with deterministic dispatch-prompt check;
writing-plans-no-spec-conversational 2/2 pass;
triggering-writing-plans canary 3/3.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/subagent-driven-development/SKILL.md

@@ -86,6 +86,10 @@ digraph process {
 }
 ```
 
+## Spec Context
+
+If the plan's header cites a spec (`**Spec:** <path>`), read it once during plan extraction. Plans reference requirements rather than restating them — when a task cites a spec section, paste that section's text into the implementer and spec-reviewer prompts along with the task text. Implementer subagents never read the spec file themselves; the spec reviewer may additionally read it at the cited path (its prompt says so).
+
 ## Model Selection
 
 Use the least powerful model that can handle each role to conserve cost and increase speed.