fold redundant test_coverage.sh into the gate

`ci/test.sh` now runs `flutter test --coverage`, so
`ci/test_coverage.sh` was just re-running the same tests plus an
optional `lcov --summary` that needs `lcov` installed (it wasn't,
on at least this machine). Removing it.

- ci/test_coverage.sh: deleted.
- Makefile: drop the `coverage` target (it only wrapped the dead
  script). Fix a stale `coverage/floor.txt` reference in the
  `coverage-gate` help text — the floor lives in pubspec.yaml now.
- .gitea/workflows/test.yml: replace the test_coverage.sh invocation
  with ci/coverage_gate.sh, so CI enforces the same floor as the
  pre-push hook (defense in depth).

Co-Authored-By: Claude <noreply@anthropic.com>

ci/test_coverage.sh

@@ -1,14 +0,0 @@
-#!/usr/bin/env bash
-# Generate + summarize lcov coverage. No thresholds yet (see plan's
-# "Open questions deferred" — we let the suite run for a week of real
-# commits before setting hard gates that would just need tuning).
-set -euo pipefail
-cd "$(dirname "$0")/.."
-
-echo "==> flutter test --coverage"
-flutter test --coverage
-
-if command -v lcov >/dev/null 2>&1; then
-  echo "==> lcov summary"
-  lcov --summary coverage/lcov.info
-fi