feat: terminal launcher diagnostics and mutation preflight (Closes #556)

Add categorize-and-probe helpers for shell spawn failures, expose
gitea_diagnose_terminal, and fail closed on mutation capability resolve
when the terminal launcher is unhealthy (BLOCKED + DIAGNOSE). Document
the operator path in the workflow runbooks.

native_mcp_preference.py

@@ -8,6 +8,9 @@ available unless explicit recovery-mode proof is supplied.
 from __future__ import annotations
 
 import re
+import os
+import shutil
+import subprocess
 from typing import Any
 
 from reviewer_fallback import LOCAL_GITEA_SCRIPT_NAMES
@@ -366,4 +369,154 @@ def assess_gitea_operation_path(
                 else "proceed with native MCP"
             )
         ),
-    }
+    }
+
+
+def default_terminal_probe_command() -> list[str]:
+    return ["git", "--version"] if shutil.which("git") else ["echo", "ok"]
+
+
+def _resolve_executable(cwd: str | None, executable: str | None) -> str | None:
+    if not executable:
+        return None
+    if os.path.isabs(executable):
+        return executable if os.path.exists(executable) and os.access(executable, os.X_OK) else None
+    if "/" in executable or "\\" in executable:
+        target_cwd = cwd or os.getcwd()
+        full_path = os.path.abspath(os.path.join(target_cwd, executable))
+        return full_path if os.path.exists(full_path) and os.access(full_path, os.X_OK) else None
+    return shutil.which(executable)
+
+
+def diagnose_terminal_failure(cwd: str | None, command: list[str]) -> dict[str, Any]:
+    """Inspect and categorize command spawn failures (#556)."""
+    diagnostics = {
+        "cwd": cwd,
+        "command": command,
+        "cwd_exists": True,
+        "cwd_is_dir": True,
+        "shell_exists": True,
+        "executable_exists": True,
+        "resolved_executable": None,
+        "error_type": "session launcher failure",
+        "error_msg": (
+            "Subprocess spawn failed despite valid CWD and executable. This may be due "
+            "to sandboxing, permission restrictions, or system resource limits."
+        ),
+    }
+
+    if cwd:
+        if not os.path.exists(cwd):
+            diagnostics["cwd_exists"] = False
+            diagnostics["error_type"] = "missing cwd"
+            diagnostics["error_msg"] = f"Current working directory does not exist: {cwd}"
+            return diagnostics
+        if not os.path.isdir(cwd):
+            diagnostics["cwd_is_dir"] = False
+            diagnostics["error_type"] = "cwd is not a directory"
+            diagnostics["error_msg"] = f"Current working directory is not a directory: {cwd}"
+            return diagnostics
+
+    exe = command[0] if command else None
+    if exe:
+        resolved_exe = _resolve_executable(cwd, exe)
+        diagnostics["resolved_executable"] = resolved_exe
+        if not resolved_exe:
+            diagnostics["executable_exists"] = False
+            if "venv" in exe or "pytest" in exe:
+                diagnostics["error_type"] = "missing runtime wrapper"
+                diagnostics["error_msg"] = (
+                    "Virtual environment runtime wrapper/executable not found or not "
+                    f"executable: {exe}"
+                )
+            else:
+                diagnostics["error_type"] = "missing executable"
+                diagnostics["error_msg"] = f"Executable not found in PATH or CWD: {exe}"
+            return diagnostics
+
+    # Check common shell availability
+    has_any_shell = False
+    for shell_path in ("/bin/sh", "/bin/zsh", "/bin/bash"):
+        if os.path.exists(shell_path) and os.access(shell_path, os.X_OK):
+            has_any_shell = True
+            break
+    if not has_any_shell:
+        diagnostics["shell_exists"] = False
+        diagnostics["error_type"] = "missing shell"
+        diagnostics["error_msg"] = "No standard system shell (/bin/sh, /bin/zsh, /bin/bash) is available or executable."
+        return diagnostics
+
+    return diagnostics
+
+
+def probe_terminal_spawn(
+    cwd: str | None = None,
+    command: list[str] | None = None,
+) -> dict[str, Any]:
+    """Proactively probe command spawning to detect launcher health (#556)."""
+    probe_cmd = command or default_terminal_probe_command()
+    diag = diagnose_terminal_failure(cwd, probe_cmd)
+    if diag["error_type"] != "session launcher failure":
+        return {
+            "healthy": False,
+            "error_type": diag["error_type"],
+            "error_msg": diag["error_msg"],
+            "cwd": cwd,
+            "command": probe_cmd,
+            "diagnostics": diag,
+        }
+
+    try:
+        proc = subprocess.run(
+            probe_cmd,
+            capture_output=True,
+            text=True,
+            cwd=cwd,
+            timeout=5,
+        )
+        if proc.returncode == 0:
+            return {
+                "healthy": True,
+                "error_type": None,
+                "error_msg": "",
+                "cwd": cwd,
+                "command": probe_cmd,
+                "diagnostics": diag,
+            }
+        # Non-zero status still proves the launcher spawned the process.
+        return {
+            "healthy": True,
+            "error_type": None,
+            "error_msg": "",
+            "cwd": cwd,
+            "command": probe_cmd,
+            "exit_code": proc.returncode,
+            "diagnostics": diag,
+        }
+    except FileNotFoundError:
+        return {
+            "healthy": False,
+            "error_type": diag["error_type"],
+            "error_msg": diag["error_msg"],
+            "cwd": cwd,
+            "command": probe_cmd,
+            "diagnostics": diag,
+        }
+    except subprocess.TimeoutExpired as exc:
+        return {
+            "healthy": False,
+            "error_type": "probe timeout",
+            "error_msg": f"Subprocess probe timed out after {exc.timeout} seconds.",
+            "cwd": cwd,
+            "command": probe_cmd,
+            "diagnostics": diag,
+        }
+    except Exception as exc:
+        return {
+            "healthy": False,
+            "error_type": diag["error_type"],
+            "error_msg": f"Subprocess spawn failed with exception: {exc}",
+            "cwd": cwd,
+            "command": probe_cmd,
+            "diagnostics": diag,
+        }