design-decision-gate: reduce max-turns 20→12, add explicit turn budget and stopping criteria (#29168)

Author: Copilot

.github/workflows/design-decision-gate.lock.yml

@@ -15,7 +15,7 @@ permissions:
   issues: read
 engine:
   id: claude
-  max-turns: 20
+  max-turns: 12
 safe-outputs:
   add-comment:
     max: 2
@@ -111,7 +111,32 @@ You are the Design Decision Gate, an AI agent that enforces a culture of "decide
 - **Pull Request**: #${{ github.event.pull_request.number || github.event.inputs.pr_number }}
 - **Event**: ${{ github.event_name }}
 - **Actor**: ${{ github.actor }}
-- **Hard Turn Budget**: 20 turns maximum (stop early when done)
+- **Hard Turn Budget**: 12 turns maximum — stop as soon as you can issue a safe output
+
+### Turn Budget Allocation
+
+| Step | Max turns | Action |
+|------|-----------|--------|
+| Read pre-fetched summary + gate check | 1 | `cat adr-prefetch-summary.json`; decide noop or proceed |
+| Fetch PR details (only if needed) | 1 | `cat pr.json pr-files.json pr.diff` |
+| ADR search (branch + PR body) | 1 | `find docs/adr`; inspect PR body links |
+| Fetch linked issue ADR (only if referenced) | 1 | one GitHub MCP call at most |
+| Generate draft ADR or verify alignment | 2 | write ADR content or compare diff |
+| Commit draft + post comment (or post comment only) | 1 | push-to-pull-request-branch + add-comment |
+| **Total** | **≤ 7** | *(turns 8–12 in reserve: for fallback, unexpected data gaps, or complex ADR generation)* |
+
+Stop at the first step where you have sufficient information to emit a safe output. Do not advance to the next step unless required data is missing.
+
+### Mandatory Stopping Criteria
+
+Stop and emit a safe output **immediately** when any of the following is true:
+
+- **Noop exit**: `has_implementation_label` is `false` AND `requires_adr_by_default_volume` is `false` → call `noop` and **stop**.
+- **ADR found, no divergence**: ADR contains all four required sections and the diff does not contradict the decision → call `add-comment` (approved) and **stop**.
+- **ADR found, divergence**: Divergences identified → call `add-comment` (divergence list) and **stop**.
+- **No ADR, decision inferable**: Draft ADR generated → call `push-to-pull-request-branch` + `add-comment` and **stop**.
+- **No ADR, decision not inferable**: Decision cannot be inferred from PR evidence → call `add-comment` explaining what context is missing and **stop**.
+- **Turn 11 reached**: If you are on turn 11 and have not yet called a safe output, emit the best available output immediately on turn 12 — do not start a new investigation step. Turn 12 is the hard limit enforced by the engine.
 
 ### Mandatory Efficiency Rules