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>

docs/ARCHITECTURE.md

@@ -193,6 +193,16 @@ parent dispatches, modes/ holds per-flag behavior (`power.md`, `all.md`,
 checkpoint.json schemas that are read only when the corresponding output
 file is being written.
 
+`workflows/plan-phase.md`, `workflows/execute-phase.md`, and the
+`gsd-planner` / `gsd-executor` agent definitions apply the same discipline
+to their MVP-only reference bodies — `planner-mvp-mode.md`,
+`user-story-template.md`, `skeleton-template.md`, and `execute-mvp-tdd.md`
+are referenced for the planner/executor to Read only on MVP,
+Walking-Skeleton, or MVP+TDD paths, rather than eagerly `@`-imported, so
+non-MVP runs do not pay their context cost (guards against the "`@`-import
+behind a conditional still loads eagerly" leak; see #720). The dedicated
+`mvp-phase` workflow keeps its eager imports, since it is always MVP.
+
 ### Agents (`agents/*.md`)
 
 Specialized agent definitions with frontmatter specifying: