feat(discuss): add discussion manifest for mechanical process verification

Closes the remaining gap in multi-milestone enforcement: the code
previously validated only the END STATE (files exist) but not the
PROCESS (each gate was presented to the user).

New mechanism:
- discuss.md instructs the LLM to write .gsd/DISCUSSION-MANIFEST.json
  after EACH Phase 3 gate decision, tracking gates_completed vs total
- checkAutoStartAfterDiscuss() Gate 4: BLOCKS auto-start if
  gates_completed < total (not just a warning)
- Manifest is deleted after auto-start (only needed during discussion)
- Single-milestone discussions don't use manifest (backward-compatible)
- DISCUSSION-MANIFEST.json added to baseline gitignore patterns

This creates a three-layer enforcement:
  Layer 1 (Prompt): ask_user_questions calls at each gate
  Layer 2 (Files):  CONTEXT.md/DRAFT/directory existence check
  Layer 3 (Manifest): gates_completed == total process verification

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/gitignore.ts

@@ -23,6 +23,7 @@ const BASELINE_PATTERNS = [
   ".gsd/metrics.json",
   ".gsd/completed-units.json",
   ".gsd/STATE.md",
+  ".gsd/DISCUSSION-MANIFEST.json",
 
   // ── OS junk ──
   ".DS_Store",

src/resources/extensions/gsd/guided-flow.ts

@@ -88,6 +88,38 @@ export function checkAutoStartAfterDiscuss(): boolean {
     } catch { /* non-fatal — PROJECT.md parsing failure shouldn't block auto-start */ }
   }
 
+  // Gate 4: Discussion manifest process verification (multi-milestone only)
+  // The LLM writes DISCUSSION-MANIFEST.json after each Phase 3 gate decision.
+  // If the manifest exists but gates_completed < total, the LLM hasn't finished
+  // presenting all readiness gates to the user — block auto-start.
+  const manifestPath = join(basePath, ".gsd", "DISCUSSION-MANIFEST.json");
+  if (existsSync(manifestPath)) {
+    try {
+      const manifest = JSON.parse(readFileSync(manifestPath, "utf-8"));
+      const total = typeof manifest.total === "number" ? manifest.total : 0;
+      const completed = typeof manifest.gates_completed === "number" ? manifest.gates_completed : 0;
+
+      if (total > 1 && completed < total) {
+        // Discussion not complete — block auto-start until all gates are done
+        return false;
+      }
+
+      // Cross-check manifest milestones against PROJECT.md if available
+      if (projectFile) {
+        const projectContent = readFileSync(projectFile, "utf-8");
+        const projectIds = parseMilestoneSequenceFromProject(projectContent);
+        const manifestIds = Object.keys(manifest.milestones ?? {});
+        const untracked = projectIds.filter(id => !manifestIds.includes(id));
+        if (untracked.length > 0) {
+          ctx.ui.notify(
+            `Discussion manifest missing gates for: ${untracked.join(", ")}`,
+            "warning",
+          );
+        }
+      }
+    } catch { /* malformed manifest — warn but don't block */ }
+  }
+
   // Draft promotion cleanup: if a CONTEXT-DRAFT.md exists alongside the new
   // CONTEXT.md, delete the draft — it's been consumed by the discussion.
   try {
@@ -95,6 +127,9 @@ export function checkAutoStartAfterDiscuss(): boolean {
     if (draftFile) unlinkSync(draftFile);
   } catch { /* non-fatal — stale draft doesn't break anything, CONTEXT.md wins */ }
 
+  // Cleanup: remove discussion manifest after auto-start (only needed during discussion)
+  try { unlinkSync(manifestPath); } catch { /* may not exist for single-milestone */ }
+
   pendingAutoStart = null;
   startAuto(ctx, pi, basePath, false, { step }).catch(() => {});
   return true;

src/resources/extensions/gsd/prompts/discuss.md

@@ -227,6 +227,27 @@ For each remaining milestone **one at a time, in sequence**, use `ask_user_quest
 
 Each context file (full or draft) should be rich enough that a future agent encountering it fresh — with no memory of this conversation — can understand the intent, constraints, dependencies, what this milestone unlocks, and what "done" looks like.
 
+#### Milestone Gate Tracking (MANDATORY for multi-milestone)
+
+After EVERY Phase 3 gate decision, immediately write or update `.gsd/DISCUSSION-MANIFEST.json` with the cumulative state. This file is mechanically validated by the system before auto-mode starts — if gates are incomplete, auto-mode will NOT start.
+
+```json
+{
+  "primary": "M001",
+  "milestones": {
+    "M001": { "gate": "discussed", "context": "full" },
+    "M002": { "gate": "discussed", "context": "full" },
+    "M003": { "gate": "queued",    "context": "none" }
+  },
+  "total": 3,
+  "gates_completed": 3
+}
+```
+
+Write this file AFTER each gate decision, not just at the end. Update `gates_completed` incrementally. The system reads this file and BLOCKS auto-start if `gates_completed < total`.
+
+For single-milestone projects, do NOT write this file — it is only for multi-milestone discussions.
+
 #### Phase 4: Finalize
 
 7. Update `.gsd/STATE.md`