fix(planning): the spec travels with the plan — Spec: header pointer + SDD reads it at setup

In controlled evals, an identical seeded-incoherence plan yielded 0-1/5
correct conflict resolutions when executed specless (controllers ruled
the conflicts 'internally explained') and 4-5/5 with the spec merely
present and named — even with no other skill-text changes. Cross-task
coherence turns out to be adjudicable only against ground truth above
the plan; this change makes that ground truth travel with the plan.

Claude-Session: https://claude.ai/code/session_0185AJr98gHx5EmwqNeft4Sy

skills/subagent-driven-development/SKILL.md

@@ -140,7 +140,10 @@ a ledger file, not only in todos.
   that happens, recover from `git log`.
 
 Read the plan once, note its context and Global Constraints, and create a
-todo per task.
+todo per task. If the plan names a Spec, read that too: the spec is the
+authority the plan argues from, and conflicts inside the plan resolve
+against it. A plan with no reachable spec gets a ledger note saying so —
+rulings made without one are provisional.
 
 Before dispatching Task 1, scan the plan once for conflicts:
 

skills/writing-plans/SKILL.md

@@ -66,6 +66,9 @@ independently testable deliverable.
 
 **Tech Stack:** [Key technologies/libraries]
 
+**Spec:** [path to the spec/design doc this plan implements — the plan
+argues from the spec, so the spec travels with it; executors read both]
+
 ## Global Constraints
 
 [The spec's project-wide requirements — version floors, dependency limits,