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">