fix(ci): the settings token could NEVER have been stored -- Gitea reserves the GITEA_ prefix

The drift detector merged in #11 was inert and would have stayed inert silently.

WHAT HAPPENED. infra/gitea/set-ci-token.sh minted a token and PUT it to the repo Actions
secret GITEA_SETTINGS_TOKEN. Gitea RESERVES the `GITEA_` prefix for secret names -- exactly
as GitHub reserves `GITHUB_` -- and rejects that PUT with:

    HTTP 400  {"message":"invalid secret name"}

The script sent the PUT to /dev/null and printed "==> stored." unconditionally, and
`curl -sS` exits 0 on an HTTP error, so it reported success for a store that never happened.
Measured afterwards: the token existed in the user's token list, and the repo's secret list
was EMPTY. ci/gitea-settings.sh would have skipped on every CI run forever, and with
REQUIRE_GITEA_SETTINGS=true it would instead have BLOCKED EVERY PR -- including the one
fixing it.

Three changes:
  * the secret is SETTINGS_API_TOKEN; ci/gitea-settings.sh reads it, keeping
    GITEA_SETTINGS_TOKEN as a local-run fallback;
  * set-ci-token.sh checks the PUT's HTTP status AND reads the secret back, refusing to claim
    success otherwise. That missing verification is the actual defect -- the wrong name was
    just what it hid;
  * gate.yml maps the secret and ENABLES REQUIRE_GITEA_SETTINGS.

WHY ENABLING IT HERE IS SAFE: this PR's own CI run is the proof. If the stage were still
skipping, REQUIRE turns that into a failure and this PR goes RED rather than merging. Green
means the token resolved and live Gitea matched infra/gitea/expected-branch-protection.json.

Verified locally before pushing:
  REQUIRE=true, token present -> MERGE GATE: PASS
  REQUIRE=true, no token      -> gitea-settings SKIPPED -> BLOCKED (not a silent skip)

The secret is already stored and read back as present, so CI has what it needs.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

ci/gitea-settings.sh

@@ -56,14 +56,20 @@ log "Stage: Gitea platform settings match the declaration"
 DECL="${REPO_ROOT}/infra/gitea/expected-branch-protection.json"
 GITEA_URL="${GITEA_URL:-https://gitea.home.keithjackson.dev}"
 GITEA_REPO="${GITEA_REPO:-Home-Network/home-network-k3s-impl}"
-TOKEN="${GITEA_SETTINGS_TOKEN:-}"
+# SETTINGS_API_TOKEN, not GITEA_SETTINGS_TOKEN. Gitea RESERVES the `GITEA_` prefix for
+# secret names and rejects a PUT of one with HTTP 400 "invalid secret name" — the same way
+# GitHub reserves `GITHUB_`. The original name could therefore never be stored, and because
+# infra/gitea/set-ci-token.sh sent that PUT to /dev/null and printed "stored" unconditionally,
+# it looked like it had worked. A token was minted, no secret existed, and this stage would
+# have skipped forever. GITEA_SETTINGS_TOKEN is still honoured for a local run.
+TOKEN="${SETTINGS_API_TOKEN:-${GITEA_SETTINGS_TOKEN:-}}"
 
 [ -f "$DECL" ] || die "declaration missing: ${DECL}"
 require_tool jq
 require_tool curl
 
 if [ -z "$TOKEN" ]; then
-  skip "GITEA_SETTINGS_TOKEN is not set — cannot read the live Gitea rule."
+  skip "SETTINGS_API_TOKEN is not set — cannot read the live Gitea rule."
   skip "  Branch protection and runner registration are therefore UNVERIFIED. They were"
   skip "  wrong, unnoticed, for a month (2026-09-29). Wire it: infra/gitea/set-ci-token.sh"
   summary "- gitea-settings: **SKIPPED — no token** (protection + runners UNVERIFIED; not a pass)"