fix: clarify capability preflight lifetime across safe reads (Closes #470)

Add preflight_contract helpers with explicit re-resolve error messages,
document the read-only tool set and consumption rules, and extend tests
for close_pr sequencing and task replacement.

Validation: ./venv/bin/python -m pytest tests/test_preflight_read_survival.py tests/test_mcp_server.py::TestPreflightVerification -q

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

gitea_mcp_server.py

@@ -502,7 +502,7 @@ def verify_preflight_purity(
         )
     if not _preflight_capability_called:
         raise RuntimeError(
-            "Pre-flight order violation: Task capability (gitea_resolve_task_capability) has not been resolved (fail closed)"
+            preflight_contract.format_missing_capability_error(task)
         )
     if (
         task is not None
@@ -510,9 +510,9 @@ def verify_preflight_purity(
         and task != _preflight_resolved_task
     ):
         raise RuntimeError(
-            "Pre-flight task mismatch: "
-            f"resolved '{_preflight_resolved_task}' but mutation requires "
-            f"'{task}' (fail closed)"
+            preflight_contract.format_task_mismatch_error(
+                _preflight_resolved_task, task
+            )
         )
 
     ctx = _resolve_author_mutation_context(worktree_path)
@@ -600,6 +600,7 @@ import issue_lock_adoption  # noqa: E402
 import merge_approval_gate  # noqa: E402
 import already_landed_reconcile  # noqa: E402
 import author_mutation_worktree  # noqa: E402
+import preflight_contract  # noqa: E402
 import issue_claim_heartbeat  # noqa: E402
 import issue_work_duplicate_gate  # noqa: E402
 import reviewer_pr_lease  # noqa: E402
@@ -6886,7 +6887,9 @@ def gitea_acquire_conflict_fix_lease(
         task_capability_map.required_permission("comment_issue"))
     if blocked:
         return blocked
-    verify_preflight_purity(remote, worktree_path=worktree_path)
+    verify_preflight_purity(
+        remote, worktree_path=worktree_path, task="comment_issue"
+    )
     comments = _list_pr_lease_comments(
         pr_number,
         remote=remote,