refactor(testing): rename tdd_bug/tdd_bug_N tags to tdd_issue/tdd_issue_N

Rename the TDD tag system from tdd_bug/tdd_bug_<N> to tdd_issue/tdd_issue_<N>
across the entire codebase. The tdd_expected_fail tag is unchanged.

The TDD expected-failure workflow is not limited to bug fixes — it applies
equally to any issue type (features, tasks, refactors). The _bug suffix was
misleading and narrowed the perceived scope. The new _issue suffix accurately
reflects that the TDD tagging system applies to any Forgejo issue.

Changes span 92 files:
- features/environment.py: validate_tdd_tags(), should_invert_result(), and
  apply_tdd_inversion() updated — regex, variables, error messages
- robot/tdd_expected_fail_listener.py: _validate_tdd_tags(), _should_invert_result(),
  start_test(), end_test() updated consistently
- 33 Behave .feature files: all @tdd_bug/@tdd_bug_<N> tags renamed
- 29 Robot .robot files: all tdd_bug/tdd_bug_<N> tags renamed
- 3 Robot fixture files renamed (tdd_bug_alone, tdd_missing_tdd_bug,
  tdd_expected_fail_missing_bug_n) with content and references updated
- Tag validation tests and helpers updated (function names, command dispatch
  keys, output strings, fixture references)
- CONTRIBUTING.md: section renamed from 'TDD Bug Test Tags' to
  'TDD Issue Test Tags', all tag references and examples updated
- noxfile.py: comment references updated
- Step definition files, mock helpers, and benchmark files: docstring
  references updated

ISSUES CLOSED: #965

benchmarks/tdd_actor_list_validation_bench.py

@@ -1,4 +1,4 @@
-"""ASV benchmarks for TDD Bug #592 — actor list validation error.
+"""ASV benchmarks for TDD Issue #592 — actor list validation error.
 
 Measures the performance of ``ActorRegistry.list_actors()`` when a provider
 has a default model containing ``/`` characters.  Establishes a baseline
@@ -33,7 +33,7 @@ from mocks.fake_provider import FakeProviderInfo, make_registry  # noqa: E402
 
 
 class TDDActorListValidationSuite:
-    """Benchmark actor list with multi-slash provider models (TDD bug #592)."""
+    """Benchmark actor list with multi-slash provider models (TDD issue #592)."""
 
     timeout = 60
 

benchmarks/tdd_session_create_di_bench.py

@@ -1,4 +1,4 @@
-"""ASV benchmarks for TDD Bug #570 — session create CLI throughput.
+"""ASV benchmarks for TDD Issue #570 — session create CLI throughput.
 
 Measures the performance of the session create CLI command path to establish
 a baseline before and after the DI bug fix.  Uses a mocked service so the
@@ -15,7 +15,7 @@ from cleveragents.cli.commands.session import app as session_app
 
 
 class TDDSessionCreateDISuite:
-    """Benchmark session create command throughput (TDD bug #570 baseline)."""
+    """Benchmark session create command throughput (TDD issue #570 baseline)."""
 
     def setup(self) -> None:
         self._svc = MagicMock()

benchmarks/tdd_session_list_di_bench.py

@@ -1,4 +1,4 @@
-"""ASV benchmarks for TDD Bug #554 — session list CLI throughput.
+"""ASV benchmarks for TDD Issue #554 — session list CLI throughput.
 
 Measures the performance of the session list CLI command path to establish
 a baseline before and after the DI bug fix.  Uses a mocked service so the
@@ -15,7 +15,7 @@ from cleveragents.cli.commands.session import app as session_app
 
 
 class TDDSessionListDISuite:
-    """Benchmark session list command throughput (TDD bug #554 baseline)."""
+    """Benchmark session list command throughput (TDD issue #554 baseline)."""
 
     def setup(self) -> None:
         self._svc = MagicMock()