refactor(#1852): lazy-split plan-phase.md into steps/ (#1934)

* refactor(#1852): lazy-split plan-phase.md into steps/

Extract 3 self-contained, rarely-hit sections into gsd-core/workflows/plan-phase/steps/
via lazy 'Read and execute' pointers (mirrors execute-phase/steps/, ADR-1610 progressive
disclosure — no eager @-import): closed-phase-gate (1.5), prd-express-path (3.5),
windows-troubleshooting. Byte-invariant: plan-phase.md 94459 -> 89775 (-4684), each step
< 32 KiB anchor, resolved instructions unchanged.

Scope note: only 3 sections were extractable. plan-phase.md is guarded by a dense net of
content-presence tests (plan-bounce/enh-3209/phase6-planning-capabilities assert specific
sections/flags inline) that block extracting the larger blocks without also refactoring
those tests — durable low-80s headroom is deferred pending maintainer re-scope (issue #1852).

Cascade: size:baseline regen, 16 golden-install-parity fixtures regen, INVENTORY in sync.
Full plan-phase test surface green (4129/4130, 0 fail); lint:ci exit 0.

* docs(changeset): Changed fragment for #1934 (plan-phase lazy-split)

* docs(changeset): mark #1934 fragment docs-exempt (internal workflow refactor)

* fix(#1852): add gsd_run launcher preamble to prd-express-path step

The extracted prd-express-path.md calls gsd_run but the canonical launcher
preamble lived in the parent plan-phase.md — runtime-launcher-parity (#373)
walks workflows/ recursively and requires every .md using gsd_run to carry
exactly one preamble + the $HOME/.claude fallback arm (same as the existing
execute-phase/steps/ files). Injected via scripts/sync-runtime-launcher.cjs;
golden fixtures + size baseline regenerated. plan-phase.md unchanged (89775).

---------

Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

tests/fixtures/golden-install-parity/opencode.json

@@ -333,7 +333,10 @@
   "gsd-core/workflows/note.md": "0d1374f2a2257858",
   "gsd-core/workflows/pause-work.md": "c9f0b8826845dda7",
   "gsd-core/workflows/plan-milestone-gaps.md": "5ec459734bf7570c",
-  "gsd-core/workflows/plan-phase.md": "6db3e766b7703874",
+  "gsd-core/workflows/plan-phase.md": "03745f51d63d99b8",
+  "gsd-core/workflows/plan-phase/steps/closed-phase-gate.md": "4099ef6d0868de60",
+  "gsd-core/workflows/plan-phase/steps/prd-express-path.md": "fa7f29c323f4cb57",
+  "gsd-core/workflows/plan-phase/steps/windows-troubleshooting.md": "e9de7a96bbfff261",
   "gsd-core/workflows/plan-review-convergence.md": "4f884d793d72d3fd",
   "gsd-core/workflows/plant-seed.md": "507e886d0f70d84c",
   "gsd-core/workflows/pr-branch.md": "3abaa18246facdc3",