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_write_tool_gates.py

@@ -1,3 +1,8 @@
+import sys
+from pathlib import Path
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+from mutation_profile_fixture import install_deterministic_remote_urls  # noqa: E402
+install_deterministic_remote_urls()
 """Regression tests: issue-write tool gates match gitea_resolve_task_capability (#69)."""
 import json
 import os
@@ -41,6 +46,7 @@ CONFIG = {
                 "gitea.read", "gitea.issue.create", "gitea.issue.close",
                 "gitea.issue.comment"],
             "forbidden_operations": [],
+            "allowed_repositories": ["Scaled-Tech-Consulting/Gitea-Tools", "Example-Org/Example-Repo", "913443/eAgenda"],
             "execution_profile": "full-author",
         },
     },