refactor(#720): lazy-load MVP-only reference bodies on non-MVP runs (#746)

* refactor(#720): lazy-load MVP-only reference bodies (eager @-import → gated Read)

Convert eager @-imports of MVP-only reference bodies into lazy "Read" instructions
gated on MVP_MODE / WALKING_SKELETON / MVP+TDD, so non-MVP planning/execution runs
no longer pull MVP guidance into context. Covers both the workflow files and the
planner/executor agent definitions (the dominant context-cost path):

- workflows/plan-phase.md: planner-mvp-mode.md + skeleton-template.md (L146/936/937/941)
- workflows/execute-phase.md: execute-mvp-tdd.md halt-report ref, now gated on gate-trip (L191)
- agents/gsd-planner.md: planner-mvp-mode.md, user-story-template.md, skeleton-template.md
- agents/gsd-executor.md: execute-mvp-tdd.md

The dedicated always-MVP mvp-phase workflow keeps its eager imports (intentional).
Behaviour is unchanged; non-MVP runs simply carry less loaded context. Adds a
regression guard mirroring the discuss-phase lazy-load test, and documents the
conformance in docs/ARCHITECTURE.md.

Refs #720

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs(#720): add changeset fragment (pr #746)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

gsd-core/workflows/execute-phase.md

@@ -188,7 +188,7 @@ if [ "$MVP_MODE" = "true" ] && [ "$TDD_MODE" = "true" ]; then
   fi
 fi
 ```
-Pure doc-only / config-only / test-only tasks return `is_behavior_adding=false` and are exempt. See `execute-mvp-tdd.md` for the halt report format.
+Pure doc-only / config-only / test-only tasks return `is_behavior_adding=false` and are exempt. When the gate trips, Read `~/.claude/gsd-core/references/execute-mvp-tdd.md` for the exact halt report format.
 </step>
 
 <step name="check_blocking_antipatterns" priority="first">

gsd-core/workflows/plan-phase.md

@@ -143,7 +143,7 @@ fi
 ```
 
 When `WALKING_SKELETON=true`:
-- Planner is instructed to produce `SKELETON.md` in the phase directory alongside `PLAN.md`. The template lives at `@~/.claude/gsd-core/references/skeleton-template.md`.
+- Planner is instructed to produce `SKELETON.md` in the phase directory alongside `PLAN.md`. The template lives at `~/.claude/gsd-core/references/skeleton-template.md` — the planner reads it when producing SKELETON.md (lazy; not loaded on non-skeleton runs).
 - The plan must scaffold project + routing + one real DB read/write + one real UI interaction + dev deployment — the thinnest possible end-to-end working slice.
 
 **Interaction with `--prd <filepath>`.** `--mvp` and `--prd` compose. The PRD express path (Step 3.5) creates `CONTEXT.md` from the PRD file and continues to research; the Walking Skeleton gate fires independently from the conditions above. When both are active on Phase 1 of a new project, the planner receives `WALKING_SKELETON=true` and PRD-derived context simultaneously — the PRD informs *what the skeleton should prove*. No precedence is needed; the two signals are orthogonal. See [`references/mvp-concepts.md`](../references/mvp-concepts.md) for the broader interaction map.
@@ -933,12 +933,12 @@ Each TDD plan gets one feature with RED/GREEN/REFACTOR gate sequence.
 </tdd_mode_active>
 ` : ''}
 
-**MVP_MODE:** ${MVP_MODE} (when true, follow vertical-slice rules from `@~/.claude/gsd-core/references/planner-mvp-mode.md`; when false, ignore MVP guidance entirely.)
-**WALKING_SKELETON:** ${WALKING_SKELETON} (when true, the first deliverable must be a Walking Skeleton — produce SKELETON.md alongside PLAN.md.)
+**MVP_MODE:** ${MVP_MODE} (when true, follow vertical-slice rules from `~/.claude/gsd-core/references/planner-mvp-mode.md`; when false, ignore MVP guidance entirely.)
+**WALKING_SKELETON:** ${WALKING_SKELETON} (when true, the first deliverable must be a Walking Skeleton — Read the template at `~/.claude/gsd-core/references/skeleton-template.md` and produce SKELETON.md alongside PLAN.md.)
 
 ${MVP_MODE === 'true' ? `
 <mvp_mode_active>
-**MVP Mode is ENABLED.** Follow vertical-slice planning rules from @~/.claude/gsd-core/references/planner-mvp-mode.md. Each plan must deliver a complete vertical slice — thin end-to-end functionality rather than horizontal layers.
+**MVP Mode is ENABLED.** Read `~/.claude/gsd-core/references/planner-mvp-mode.md` now and follow its vertical-slice planning rules. Each plan must deliver a complete vertical slice — thin end-to-end functionality rather than horizontal layers.
 </mvp_mode_active>
 ` : ''}
 </planning_context>