fix(#2770): decision-coverage gate fails closed on empty arg + workflow recomputes CONTEXT_PATH in-block (#2881)

* test(#2770): empty contextPath argument must fail closed, not green-skip the decision-coverage gate

The handler conflated empty-arg (caller error) with file-missing (legitimate skip),
returning passed:true/skipped on an empty argument. Add: empty arg → passed:false;
real-path-to-absent-file → legitimate green skip preserved; omitted arg → fail closed.

* fix(#2770): decision-coverage gate fails closed on empty arg + workflow recomputes CONTEXT_PATH in-block

Handler (check-command-router.cts): split the guard — empty/missing contextPath
argument is a caller error (fail closed, passed:false, mirrors #1365); a real path
whose file genuinely does not exist keeps the legitimate green skip.

Workflow (plan-phase.md): recompute CONTEXT_PATH inside the consuming Bash block
(it was set in the step-1 init block, which does not survive into the separately-
spawned gate block — so the gate ran with an empty arg and silently green-skipped).

* chore(#2770): changeset fragment

* fix(#2770): guard workflow empty-glob case (review blocker) + update drift-guard test

The handler now fails closed on an empty contextPath arg, so the workflow's
unguarded glob (empty when a phase genuinely has no CONTEXT.md) would invoke the
gate with an empty arg → passed:false → exit 1, hard-halting the legitimate
'Continue without context' plan-phase path. Guard the empty-glob case: only run the
gate when a CONTEXT.md actually exists. Update the F1 drift-guard test (which gave
false coverage — it only checked for the ${CONTEXT_PATH} token) to assert the
in-block recompute AND the empty-glob guard.

* fix(#2770): keep plan-phase.md under ADR-857 size cap + ack emitted drift + fix drift-guard window

The workflow fix grew plan-phase.md past the ADR-857 phase-6 size cap (94519B) and
triggered emitted-attribution. Condense adjacent §13a prose/JSON to offset (net
+89B, under cap). Add tests/emitted-drift-ack.json acknowledging the residual growth.
Widen the drift-guard test window (the gate invocation is now nested in the empty-glob
guard, so the old 400-char window missed the glob recompute).

* chore(#2770): backfill changeset PR number (2881)

---------

Co-authored-by: Test <test@example.com>

gsd-core/workflows/plan-phase.md

@@ -1400,47 +1400,43 @@ If `TEXT_MODE` is true, present as a plain-text numbered list (options already s
 
 ## 13a. Decision Coverage Gate
 
-After the requirements coverage gate passes, verify that every trackable
-decision captured by discuss-phase in CONTEXT.md `<decisions>` is referenced
-by at least one plan. This is the **translation gate** from issue #2492 —
-its job is to refuse to mark a phase planned when a discuss-phase decision
-silently dropped on the way into the plans.
+Verify every trackable decision in CONTEXT.md `<decisions>` is referenced by at
+least one plan. This **translation gate** (#2492) refuses to mark a phase planned
+when a discuss-phase decision silently dropped.
 
-**Skip if** `workflow.context_coverage_gate` is explicitly set to `false`
-(absent key = enabled). Also skip if no CONTEXT.md exists for this phase
-(nothing to translate) or if its `<decisions>` block is empty.
+**Skip if** `workflow.context_coverage_gate` is `false` (absent = enabled), or
+no CONTEXT.md exists for this phase, or its `<decisions>` block is empty.
 
 ```bash
 GATE_CFG=$(gsd_run query config-get workflow.context_coverage_gate 2>/dev/null || echo "true")
 if [ "$GATE_CFG" != "false" ]; then
-  GATE_RESULT=$(gsd_run query check.decision-coverage-plan "${PHASE_DIR}" "${CONTEXT_PATH}")
-  # BLOCKING: refuse to mark phase planned when a trackable decision is uncovered.
-  # `passed: true` covers both real-pass and skipped cases (gate disabled / no CONTEXT.md /
-  # no trackable decisions). Verify-phase counterpart deliberately omits this exit-1 — that
-  # gate is non-blocking by design (review finding F15).
-  echo "$GATE_RESULT" | jq -e '(.passed // .data.passed) == true' >/dev/null || {
-    echo "$GATE_RESULT" | jq -r '(.message // .data.message // "Decision coverage gate failed.")'
-    exit 1
-  }
+  # #2770: CONTEXT_PATH from step-1 init doesn't survive into this Bash block;
+  # recompute it. Only run when a CONTEXT.md exists (handler fails closed on an
+  # empty arg, so an unguarded empty glob would halt a context-less phase).
+  CONTEXT_PATH=$(ls "${PHASE_DIR}"/*-CONTEXT.md 2>/dev/null | head -1)
+  if [ -n "$CONTEXT_PATH" ]; then
+    GATE_RESULT=$(gsd_run query check.decision-coverage-plan "${PHASE_DIR}" "${CONTEXT_PATH}")
+    # BLOCKING: refuse to mark phase planned when a trackable decision is uncovered.
+    # `passed: true` covers both real-pass and skipped cases (gate disabled / no CONTEXT.md /
+    # no trackable decisions). Verify-phase counterpart deliberately omits this exit-1 — that
+    # gate is non-blocking by design (review finding F15).
+    echo "$GATE_RESULT" | jq -e '(.passed // .data.passed) == true' >/dev/null || {
+      echo "$GATE_RESULT" | jq -r '(.message // .data.message // "Decision coverage gate failed.")'
+      exit 1
+    }
+  fi
 fi
 ```
 
 The handler returns JSON:
 ```json
-{
-  "passed": true,
-  "skipped": false,
-  "total":  2,
-  "covered": 2,
-  "uncovered": [ { "id": "D-01", "text": "...", "category": "..." } ],
-  "message": "..."
-}
+{ "passed": true, "skipped": false, "total": 2, "covered": 2,
+  "uncovered": [{ "id": "D-01", "text": "...", "category": "..." }], "message": "..." }
 ```
 
 **If `passed` is true (or `skipped` is true):** Display
-`✓ Decision coverage: {M}/{N} CONTEXT.md decisions covered by plans` (or
-`(skipped — gate disabled)` / `(skipped — no decisions)`) and proceed to
-step 13b.
+`✓ Decision coverage: {M}/{N} decisions covered` (or `(skipped)`) and proceed
+to step 13b.
 
 **If `passed` is false:** Display the handler's `message` block. It already
 names each uncovered decision (`D-NN | category | text`) and tells the user