fix(session): require config-backed mutation authority and workspace-bound targets (#714)

Close the #714 remediation gap left at 943d402 where env-only mutation
profiles, REMOTES Timesheet defaults treated as explicit caller input, and
machine-dependent Git remotes produced false greens.

- Fail closed when mutations lack a config-backed profile with non-empty
  allowed_repositories; env ops cannot invent mutation authority.
- Preserve omitted-vs-explicit org/repo provenance through the mutation
  gate; omitted targets bind to the verified workspace repository.
- Prefer workspace-aligned Git remotes over historical Timesheet defaults
  in MCP _resolve and CLI resolve_remote.
- Centralize deterministic config-backed mutation fixtures; migrate
  mutation tests off env-only authority without weakening security
  assertions.

Full suite: 2744 passed, 6 skipped.

tests/test_issue_content_gate.py

@@ -1,3 +1,7 @@
+import sys as _sys
+from pathlib import Path as _Path
+_sys.path.insert(0, str(_Path(__file__).resolve().parent))
+from mutation_profile_fixture import shared_mutation_env  # noqa: E402
 """Tests for the pre-create issue content gate (Issue #582)."""
 import sys
 import unittest
@@ -15,9 +19,10 @@ from issue_content_gate import (  # noqa: E402
 from mcp_server import gitea_create_issue  # noqa: E402
 
 FAKE_AUTH = "Basic dGVzdDp0ZXN0"
-ISSUE_WRITE_ENV = {
-    "GITEA_ALLOWED_OPERATIONS": "gitea.issue.create",
-}
+ISSUE_WRITE_ENV = shared_mutation_env(
+    "test-author-prgs",
+    GITEA_ALLOWED_OPERATIONS="gitea.issue.create",
+)
 
 
 class TestNormalizeAndPointers(unittest.TestCase):