feat(#1026): §5.6/ui-phase cutover — first gate dispatch (plan:pre step + blocking gate) (#1028)

Replace plan-phase.md §5.6 (a 6-branch inline UI gate) with a capability-driven
loop.render-hooks plan:pre dispatch — the FIRST gate dispatch in any workflow.
A step (ui-phase, when:workflow.ui_phase) + a new blocking gate
(when:workflow.ui_safety_gate). New ui.plan-gate check verb returns
{frontend, hasUiSpec, block}; the dispatch runs it unconditionally then fires
the active step (pipeline) or halts on the active blocking gate (manual). The
gate-handling (run check.query; halt if blocking+block) is the reusable
phase-6 template for blocking-gate cutovers.

Config semantics fixed per #1022 + maintainer call: ui_phase gates plan-time
UI-SPEC generation, ui_safety_gate gates the planning block. Common case + all
ui_phase=false cases are equivalence-preserving; the one intended change is
{ui_phase:true, ui_safety_gate:false} now auto-generating in pipelines.

Review found it broken twice (non-generic dispatch, phase-lookup divergence,
then the step-only check nested in a gate loop) — fixed; final Codex pass
verified all 8 (ui_phase,ui_safety_gate)x{pipeline,manual} cases correct.
gsd-ui-phase skill + autonomous §3a.5 untouched (§3a.5 deferred).
getRoadmapPhaseWithFallback mirrors cmdRoadmapGetPhase for lookup parity.

Closes #1026

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

capabilities/ui/capability.json

@@ -16,6 +16,7 @@
   ],
   "contributions": [],
   "gates": [
+    { "point": "plan:pre",          "check": { "query": "ui.plan-gate" },   "when": "workflow.ui_safety_gate", "blocking": true, "onError": "halt" },
     { "point": "execute:wave:post", "check": { "query": "ui.safety-gate" }, "when": "workflow.ui_safety_gate", "blocking": true, "onError": "halt" }
   ]
 }