implementation: pass the PLAN.md pointer at each handoff #7603

Parent

Refs #7587 (verification and implementation). Source: PLAN Phase 2, topic poteto-review.

Depends on

• verification/confirm: probe whether confirm reads the PLAN.md Brief #7594 Q148 probe
• pr-pipeline: admit marked epic slices in the resolve-issue lane #7596 resolve-issue admission (lane enabler)
• pr-pipeline: promote admitted items to agent-ready when their last blocker closes #7599 auto-unblock (lane enabler)

Outcome

/implementation:implement and /implementation:implement-dispatch pass the PLAN.md pointer at each
handoff, so a worker or later step can read the Brief.

Decisions

• R2 of the intent chain (D95). Origin: ours. The full verbatim pass of goal, out-of-scope and
  assumptions is a Phase 19 slice (D5).

Key interfaces

From the design's module layout: "implementation: implement-dispatch passes the Brief's goal,
out-of-scope and assumptions verbatim."

Done when

• An eval case added to the edited skill shows a dispatched worker brief that names the PLAN.md
  pointer; it fails on main and passes after.
• bash plugins/skill-quality/scripts/check-skill.sh plugins/implementation/skills exits 0.

Out of scope

• Passing the Brief's sections verbatim (Phase 19).
• Merging unattended: the PR carries do-not-merge until the Merge authority thread passes (B10).