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_commit_files_capability.py

@@ -29,6 +29,7 @@ CONFIG = {
             "allowed_operations": ["gitea.read", "gitea.repo.commit"],
             "forbidden_operations": ["gitea.pr.create"],
             "execution_profile": "commit-author",
+        "allowed_repositories": ["Example-Org/Example-Repo"],
         },
         "pr-only-author": {
             "enabled": True,
@@ -39,6 +40,7 @@ CONFIG = {
             "allowed_operations": ["gitea.read", "gitea.pr.create"],
             "forbidden_operations": ["gitea.repo.commit"],
             "execution_profile": "pr-only-author",
+        "allowed_repositories": ["Example-Org/Example-Repo"],
         },
         "reviewer-profile": {
             "enabled": True,
@@ -51,6 +53,7 @@ CONFIG = {
                 "gitea.repo.commit", "gitea.pr.create", "gitea.branch.push",
             ],
             "execution_profile": "reviewer-profile",
+        "allowed_repositories": ["Example-Org/Example-Repo"],
         },
     },
     "rules": {"allow_runtime_switching": False},