feat(#1820): spec-optional predicate rail — author probe predicates into must_haves when SPEC omits them

gsd-core/workflows/plan-phase.md

@@ -693,6 +693,19 @@ echo "✓ API surface regenerated: ${API_SURFACE_PATH}"  # injected into step 8
 
 Continue to step 8.
 
+## 7.95. Spec-less Probe Fallback (gate)
+
+When the SPEC did not supply `## Edge Coverage` / `## Prohibitions`, plan-phase runs the probe protocol
+and authors the predicates into PLAN.md `must_haves` (ADR-857 Phase 6 — the *else branch* of the
+`<downstream_consumer>` lift below). Core workflow-body substrate, not a capability rail (D-03). Runs
+after `$SPEC_FILE` (Step 7), before the gsd-planner spawn (Step 8).
+
+**Read and run** the gate + edge probe in `~/.claude/gsd-core/references/specless-probe-fallback.md`
+(§0 default-ON toggle + per-section absence via the `spec-section` helper, visibly skipping when
+disabled or no requirement IDs; §A deterministic edge probe → `$COVERAGE` when `EDGE_ABSENT`; §B
+prohibition recall in the planner). Pass `$COVERAGE` and `$SPECLESS_FALLBACK_DISABLED` into Step 8.
+
+
 ## 8. Spawn gsd-planner Agent
 
 Display banner:
@@ -769,6 +782,22 @@ ${MVP_MODE === 'true' ? `
 **MVP Mode is ENABLED.** Read `~/.claude/gsd-core/references/planner-mvp-mode.md` now and follow its vertical-slice planning rules. Each plan must deliver a complete vertical slice — thin end-to-end functionality rather than horizontal layers.
 </mvp_mode_active>
 ` : ''}
+
+<specless_probe_fallback>
+**Spec-less probe fallback** (only when step 7.95 set `EDGE_ABSENT` and/or `PROHIB_ABSENT`). The SPEC
+omitted that section — author its predicates into `must_haves` via the `<downstream_consumer>`
+else-branch below, per §A/§B/§C of `~/.claude/gsd-core/references/specless-probe-fallback.md`
+(descriptor-less prohibitions, never auto-dismiss, no silent drops).
+
+Edge coverage report (`$COVERAGE`, present when `EDGE_ABSENT`):
+
+```json
+{COVERAGE}
+```
+${SPECLESS_FALLBACK_DISABLED ? `
+**⚠ ${SPECLESS_FALLBACK_DISABLED}** — record this in the plan (a visible, recorded choice); do not generate probe predicates this run.
+` : ''}
+
 </planning_context>
 
 <downstream_consumer>
@@ -777,8 +806,8 @@ Output consumed by /gsd:execute-phase. Plans need:
 - Tasks in XML format with read_first and acceptance_criteria fields (MANDATORY on every task)
 - Verification criteria
 - must_haves for goal-backward verification
-- If the SPEC has an `## Edge Coverage` section, lift every `covered` edge's acceptance criterion into `must_haves.truths` as a plain string, and every `backstop` edge **as a structured flat-scalar marker** — an object item `{ statement: <the check>, verification: backstop }`, NOT a prose note (the verifier branches deterministically on the `verification: backstop` field; a parenthetical is unparseable — the #1110 fragility). Use a flat scalar `verification:` continuation key, never a nested object (ADR-550 #1278). At verify time a `backstop` truth the verifier cannot confirm with explicit evidence abstains → `human_needed` (reason `insufficient_spec`), never a silent pass (#1154; see `references/honest-verifier.md`). `unresolved` edges are explicit assumptions — surface them in the plan, do not silently drop them.
-- If the SPEC has a `## Prohibitions` section, lift every resolved prohibition into the `must_haves.prohibitions:` sibling block (NOT `truths` — ADR-550 D3) carrying `statement` + `status` + `verification`; unresolved prohibitions are explicit assumptions — surface them in the plan, do not silently drop them. A prohibition is a must-NOT (negative) check that belongs in its own `must_haves.prohibitions` block. Never place a must-NOT under `must_haves.truths` — that block keeps positive-observable semantics only.
+- If the SPEC has an `## Edge Coverage` section, lift every `covered` edge's acceptance criterion into `must_haves.truths` as a plain string, and every `backstop` edge **as a structured flat-scalar marker** — an object item `{ statement: <the check>, verification: backstop }`, NOT a prose note (the verifier branches deterministically on the `verification: backstop` field; a parenthetical is unparseable — the #1110 fragility). Use a flat scalar `verification:` continuation key, never a nested object (ADR-550 #1278). At verify time a `backstop` truth the verifier cannot confirm with explicit evidence abstains → `human_needed` (reason `insufficient_spec`), never a silent pass (#1154; see `references/honest-verifier.md`). `unresolved` edges are explicit assumptions — surface them in the plan, do not silently drop them. **Otherwise** (`EDGE_ABSENT`): apply the SAME lift to the fallback report `{COVERAGE}` (per §C of `references/specless-probe-fallback.md`); a SPEC-supplied section is never re-run.
+- If the SPEC has a `## Prohibitions` section, lift every resolved prohibition into the `must_haves.prohibitions:` sibling block (NOT `truths` — ADR-550 D3) with `statement`+`status`+`verification`, via the single `projectProhibitions` serializer (Hyrum — no second serializer); unresolved -> flagged assumptions, don't drop; never put a must-NOT under `truths`. **Otherwise** (`PROHIB_ABSENT`), author the recalled prohibitions into the SAME block via the SAME `projectProhibitions` contract but **descriptor-less** (no `check_*`) so each disposes flagged-unverified; never auto-dismiss. Section-level precedence + no-silent-drop equality apply (§C).
 - **"Artifacts this phase produces" section (MANDATORY)** — list every symbol this phase creates: decorators, classes, functions, CLI flags, struct/dataclass fields, new file paths. The plan-review-convergence source-grounding pass reads this section to exclude newly-created symbols from drift verification; omitting it causes new symbols to be flagged for acknowledgement.
 </downstream_consumer>