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>

.changeset/lazy-mvp-bodies-progressive-disclosure.md

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+---
+type: Changed
+pr: 746
+---
+**`/gsd:plan-phase` and `/gsd:execute-phase` no longer eagerly load MVP-only guidance on non-MVP runs** — the MVP planner rules, user-story template, Walking-Skeleton template, and MVP+TDD halt-report reference are now Read lazily by the planner/executor only when MVP / Walking-Skeleton / MVP+TDD mode is active, in both the workflow files and the `gsd-planner`/`gsd-executor` agent definitions, instead of being `@`-imported into every run. Behaviour is unchanged; non-MVP planning/execution simply carries less context. (#720)