enhance(#3907): gates report no-input instead of a verdict they never reached (#3932)

* feat(#3907): gates report no-input instead of asserting a verdict they never reached

The three stdin-reading gates bound 2 to a stdin read error only, with no arm for stdin closed at zero bytes - so empty input flowed into the detector, found nothing, and exited 1, which each module's own comment defines as a negative verdict. An unset PHASE_SECTION made the UI gate assert the phase has no UI. Empty and whitespace-only input now exit NO_INPUT, and a read error exits UNAVAILABLE rather than a locally-invented 2, both resolved through the registry and delivered by terminateNow.

The exit code was only half of it: under --json the same input emitted {detected:false}, byte-identical to the fabricated payload #3909 exists to fix, and the blocking coverage gate reads that payload. Empty input now emits the in-tree {skipped:true,reason} form with no detected key at all.

teams-status is excluded: it never reads stdin and has no invented 2, so the four-module framing in the issue and ADR is wrong. The dead root bin/lib/ui-safety-gate.cjs is deleted - no installer reference, no workflow invocation, and the live fallback chains are for other modules. Its removal restores the unit tests to the module that actually ships; they had been asserting the stale copy's two-field shape, which is why it drifted unnoticed.

* fix(#3907): drive gate tests through the process seam, and make removed-but-needed basename-precise

CONTRIBUTING requires every subprocess go through tests/helpers/process-seam.cjs; two of the three gate suites hand-rolled spawnSync while the third, added in the same change, used runNode correctly for the identical injection case. Converted the blocks this change added, leaving pre-existing ones alone.

Deleting one of two files sharing a basename made lint-removed-but-needed report 14 references that were all to the surviving canonical module - the false-positive class its own docstring names. It now matches on the deleted file's full path when a surviving file shares its basename, which is more precise rather than weaker: a genuine full-path reference still fails, and behaviour is unchanged when no basename collides. It immediately caught a docstring on this branch that spelled the deleted path.

* test(#3907): update the one existing assertion that pinned the old empty-stdin verdict

A pre-existing test asserted exit 1 on empty stdin - the defect this phase removes - and was missed because the change added new blocks without auditing existing ones pinning the old contract. Audited the rest: the other three status-1 assertions in that file all feed real input and are the genuine-negative controls that must keep returning 1, so exactly one was stale. The retired 2 is gone from the describe's contract comment too.

* chore(#3907): backfill changeset pr number

---------

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

capabilities/ai-integration/fragments/api-coverage-plan-pre.md

@@ -25,8 +25,20 @@ SCOPE="$(cat "${PHASE_DIR}"/*-PLAN.md 2>/dev/null) $(gsd_run query roadmap.get-p
 API_COVERAGE_JSON=$(printf '%s' "$SCOPE" | node gsd-core/bin/lib/api-coverage.cjs --json 2>/dev/null || echo '{"detected":false,"signals":[]}')
 ```
 
-Read `API_COVERAGE_JSON.detected`. Act on it only — do **not** pattern-match the
-prose yourself.
+The detector's exit code and `--json` payload now distinguish a real negative
+from an unexamined input (ADR-3889 Phase 3, #3907): empty/whitespace-only
+`$SCOPE` or a stdin read failure emit `{"skipped":true,"reason":"no_input"|
+"stdin_error"}` — no `detected` key at all. **Check for `skipped` before
+reading `detected`**: a `skipped` payload is not a confirmed "no API
+integration" verdict, it means the detector never examined real input. Do not
+treat it as `detected:false`. Read `API_COVERAGE_JSON.detected` only when
+`skipped` is absent — act on it only, do **not** pattern-match the prose
+yourself.
+
+**If `skipped` is `true`:** the detector could not establish a scope (empty
+`$SCOPE`) or failed to run (stdin read error). Skip the checkpoint for this
+run rather than asserting a verdict about input that was never examined; do
+not raise it with the user.
 
 **If `detected` is `false`:** this phase does not integrate an external API. Skip
 the checkpoint entirely and continue planning. Do not raise it with the user.