fix(#4011): key the TDD runtime gate on TDD_MODE alone (#4180)

* test(#4011): TDD gate keys on TDD_MODE alone, not the MVP intersection

Contract updates: no shipped line may conjoin MVP_MODE with TDD_MODE as
a gate condition, the end-of-phase escalation must not require MVP, the
executor agent's gate section triggers on TDD_MODE alone, and the gate
semantics reference loads without MVP_MODE.

* fix(#4011): key the TDD runtime gate on TDD_MODE alone

The RED-commit gate shipped as #76's MVP slice kept the paired
invocation's conjunct, so workflow.tdd_mode=true was silently inert on
every non-MVP phase, contradicting references/tdd.md's own contract.
Drops the MVP conjunct from the per-task gate and the end-of-phase
review escalation; rescopes execute-mvp-tdd.md's load condition,
gsd-executor's gate section, and mvp-concepts' intersection claim.
MVP remains free to imply TDD; the file is not renamed (stated
assumption in the PR body).

* test(#4011): scope no-conjunct detector to shell conditions; clean stale MVP+TDD phrasing

Review follow-ups: the detector now only inspects if/[ condition lines
so explanatory prose mentioning both flags cannot trip it; remaining
'under/outside MVP+TDD' phrases in execute-phase.md, the gate
reference, and docs/INVENTORY.md now describe TDD-mode semantics.

Emitted-Drift-Ack-Growth: execute-phase.md — TDD-gate decoupling comment + escalation rescoping (#4011)
Emitted-Drift-Ack-Growth: gsd-executor.md — gate section trigger rescoped to TDD_MODE alone (#4011)

* chore(#4011): backfill changeset pr number

---------

Co-authored-by: sim <sim@local>

docs/INVENTORY.md

@@ -381,7 +381,7 @@ Full roster at `gsd-core/references/*.md`. References are shared knowledge docum
 | `executor-examples.md` | Worked examples for the gsd-executor agent. |
 | `plan-checker-examples.md` | Worked example for the gsd-plan-checker agent (Scope Exceeded), moved out of the inline `<examples>` block to keep worked examples structurally separate from the agent contract, matching the other agents' reference layout. `@`-inlined at load (eager), so this costs ~0.6 KB of runtime context versus keeping it inline — a readability trade, not a size-cap remedy (#3724). |
 | `doc-conflict-engine.md` | Shared conflict-detection contract for ingest/import workflows. |
-| `execute-mvp-tdd.md` | Runtime gate semantics for execute-phase under MVP+TDD — pre-task failing-test verification, end-of-phase blocking review. |
+| `execute-mvp-tdd.md` | Runtime gate semantics for execute-phase under TDD mode — pre-task failing-test verification, end-of-phase blocking review. |
 | `mvp-concepts.md` | Cross-reference index for the six MVP-related reference files; maps each file to its purpose and which workflow loads it. |
 | `verify-mvp-mode.md` | UAT framing rules for MVP-mode phases — user-flow-first ordering, deferred technical checks, user-story-format guard. |