Defer apex-root terminal transition until feature -> main promotion (AB#3169)

[PolyphonyRequiem]

Symptom (AB#3169)

During the AB#3166 dogfood, the apex Issue's ADO state went Doing → Done
at the moment the MG was merged into the feature branch (21:26:43Z) —
before feature → main was merged. ADO then cascaded the apex's
parent items (User Story / Feature) to Done while the work was still
sitting on the feature branch and not yet on main.

Root cause

primary_completer in .conductor/registry/workflows/implement-merge-group.yaml
unconditionally fired:

polyphony validate --event implementation_complete --work-item $taskId ...
twig state Done --id $taskId ...

…for primary_router.output.primary_id. For an indivisible apex (a
plan with one task, where primary_id == apex_id == root_id), this
transitioned the apex Issue at MG → feature merge time, ahead of the
real "the work is shipped" moment (feature → main).

process-config.yaml:

Issue.implementation_complete: Done

…so the validate event triggered the cascade.

Fix

Carve out the apex-root case in primary_completer:

1. Capture $rootId = [int]'{{ workflow.input.root_id }}'.
2. Compute $isApexRoot = ($taskId -eq $rootId).
3. Apex root branch: skip the validate + state transition. Add an
   AB#3169-referencing twig note + sync only. Output deferred_apex_root: true.
4. Child task branch: unchanged. Validate + state Done as before
   (matches pre-existing rubber-duck Feature/2582 pg 4 #7 acknowledgement at line 636-641).

The apex's terminal transition is now deferred to close_mark_satisfied
(which fires polyphony validate --event item_satisfied), reached only
after promote_feature_to_main + promote_feature_pr_dispatch from
PR #375 (AB#3168) merge feature → main.

Why apex-root only

For child Tasks inside a multi-task MG, the existing carve-out comment
explicitly accepts the MG → feature timing (Tasks have no parent
cascade — they're leaves). Only the indivisible-apex case has the
Issue → User Story → Feature cascade blast radius, so only the
indivisible-apex case needs deferral.

Sister PR

This PR is a sibling to PR #375 (AB#3168) — they were filed
together as defense-in-depth follow-ups from the AB#3166 dogfood:

• PR Insert feature → main promotion before close_mark_satisfied (AB#3168) #375 (AB#3168) restructured apex-driver.yaml to actually
  promote feature → main before close. Without this PR, the apex would
  still transition prematurely.
• PR Defer apex-root terminal transition until feature -> main promotion (AB#3169) #376 (this one) stops the premature transition at MG → feature
  merge time. Without PR Insert feature → main promotion before close_mark_satisfied (AB#3168) #375, deferring would leave the apex stuck in
  Doing forever.

Both are needed for the gap to fully close.

Tests

• lint-implement-merge-group.Tests.ps1: 38/38 pass (5 new
  structural tests in new Context "primary_completer apex-root carve-out
  (AB#3169)").
• Full registry suite: 391/393 (2 pre-existing failures in
  lint-plan-level.Tests.ps1 "Policy wiring checks" — unrelated;
  stash-confirmed on main).
• conductor validate implement-merge-group.yaml: Successful.
• All other workflow lints (strict-undefined, pwsh-jinja-bareword,
  sync-after-mutation) green.

Verification path (post-merge)

After this PR + PR #375 are both on main:

1. Dispatch an indivisible apex.
2. At MG → feature merge: primary_completer emits
   deferred_apex_root: true and adds a note. ADO state stays Doing.
3. apex-driver routes: outer_loop_evaluator(complete) → apex_completion_gate_policy_router → promote_feature_to_main → promote_feature_pr_dispatch → close_mark_satisfied.
4. close_mark_satisfied fires item_satisfied. ADO state
   transitions to Done. Parent cascade runs at the correct moment.

Closes AB#3169.