feat(#93): filesystem permission pre-flight for credential configs (0600/0700)

ssh-mcp refuses to start if its config is group/world-readable (mode & 0o077 →
throws, requiring dir 0700 / file 0600). A GUI user has no idea what chmod 600
means — they just get a dead server. This checks it and offers a one-click fix.

Core (pure, POSIX-only, injectable stat/chmod so tests never touch a real file):
- permission_status(path): the ssh-mcp rule — any group/other bit set (mode & 0o077)
  is not-ok; file must be 0600, its dir 0700. Returns None on Windows (modes don't
  apply) or when the file is absent (nothing to pre-flight). Plain-language problems
  naming the offending octal mode.
- fix_permissions(path): chmod file → 0600, dir → 0700. No-op on Windows; reports
  an OSError instead of raising.
- sidecar_permission_warnings() / sidecar_permission_fix_target(): tie the check to
  #91's sidecar path resolution so it knows WHICH file to inspect. Mirror the
  sidecar-warnings shape.

GUI: a warning label + "Fix permissions" button in the stdio editor (mirrors the
removed-flag surface), shown only when the sidecar exists and is too open. Hidden
on Windows and for non-sidecar servers. Smoke-tested headlessly.

pytest green (529 passed), ruff + format clean. Closes #93. Part of epic #94.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_core.py

@@ -3424,6 +3424,116 @@ def test_sidecar_status_none_for_unknown_package():
     assert c.sidecar_warnings({"command": "npx", "args": ["other"]}) == []
 
 
+# --------------------------------------------------------------------------- #
+# Filesystem permission pre-flight (issue #93)
+# --------------------------------------------------------------------------- #
+def test_permission_status_ok_when_0600():
+    st = c.permission_status(
+        "/cfg/config.toml",
+        platform="darwin",
+        stat_mode=lambda p: {"/cfg/config.toml": 0o600, "/cfg": 0o700}.get(str(p)),
+    )
+    assert st["ok"] is True
+    assert st["mode"] == 0o600
+    assert st["problems"] == []
+
+
+def test_permission_status_blocks_group_world_readable_file():
+    st = c.permission_status(
+        "/cfg/config.toml",
+        platform="linux",
+        stat_mode=lambda p: {"/cfg/config.toml": 0o644, "/cfg": 0o755}.get(str(p)),
+    )
+    assert st["ok"] is False
+    assert st["file_ok"] is False
+    assert st["dir_ok"] is False
+    # Two plain-language problems, naming the offending octal modes.
+    text = " ".join(st["problems"])
+    assert "0644" in text and "0600" in text
+    assert "0755" in text and "0700" in text
+
+
+def test_permission_status_file_bad_dir_ok():
+    st = c.permission_status(
+        "/cfg/config.toml",
+        platform="linux",
+        stat_mode=lambda p: {"/cfg/config.toml": 0o640, "/cfg": 0o700}.get(str(p)),
+    )
+    assert st["file_ok"] is False
+    assert st["dir_ok"] is True
+    assert len(st["problems"]) == 1
+
+
+def test_permission_status_none_on_windows_and_missing_file():
+    # Windows: POSIX modes don't apply -> None (clean no-op).
+    assert c.permission_status("/cfg/config.toml", platform="win32") is None
+    # Absent file -> nothing to pre-flight.
+    assert c.permission_status("/cfg/gone.toml", platform="linux", stat_mode=lambda p: None) is None
+
+
+def test_fix_permissions_chmods_file_and_dir():
+    calls = []
+    changed, note = c.fix_permissions(
+        "/cfg/config.toml", platform="linux", chmod=lambda p, m: calls.append((str(p), m))
+    )
+    assert changed is True
+    assert ("/cfg/config.toml", 0o600) in calls
+    assert ("/cfg", 0o700) in calls
+    assert note and "0600" in note
+
+
+def test_fix_permissions_noop_on_windows():
+    calls = []
+    changed, note = c.fix_permissions(
+        "/cfg/config.toml", platform="win32", chmod=lambda p, m: calls.append((p, m))
+    )
+    assert changed is False
+    assert note is None
+    assert calls == []
+
+
+def test_fix_permissions_reports_oserror():
+    def boom(p, m):
+        raise OSError("nope")
+
+    changed, note = c.fix_permissions("/cfg/config.toml", platform="linux", chmod=boom)
+    assert changed is False
+    assert "could not change permissions" in note
+
+
+def test_sidecar_permission_warnings_over_real_path():
+    data = {"command": "npx", "args": ["-y", "ssh-mcp", "--host=h"]}
+    real = c.sidecar_path(c.SERVER_SPECS["ssh-mcp"], platform="darwin", environ={}, home="/Users/t")
+
+    def stat_mode(p):
+        return 0o644 if Path(p) == real else 0o700
+
+    warns = c.sidecar_permission_warnings(
+        data, platform="darwin", environ={}, home="/Users/t", stat_mode=stat_mode
+    )
+    assert warns and warns[0].startswith("ssh-mcp:")
+    assert "0600" in warns[0]
+    # Windows / unknown package -> nothing.
+    assert c.sidecar_permission_warnings(data, platform="win32") == []
+    assert c.sidecar_permission_warnings({"command": "npx", "args": ["other"]}) == []
+
+
+def test_sidecar_permission_warnings_quiet_when_tight():
+    data = {"command": "npx", "args": ["-y", "ssh-mcp", "--host=h"]}
+    warns = c.sidecar_permission_warnings(
+        data, platform="darwin", environ={}, home="/Users/t", stat_mode=lambda p: 0o600
+    )
+    assert warns == []
+
+
+def test_sidecar_permission_fix_target():
+    data = {"command": "npx", "args": ["-y", "ssh-mcp"]}
+    tgt = c.sidecar_permission_fix_target(data, platform="darwin", environ={}, home="/Users/t")
+    assert tgt is not None and tgt.name == "config.toml"
+    assert c.sidecar_permission_fix_target(data, platform="win32") is None
+    assert c.sidecar_permission_fix_target({"command": "npx", "args": ["other"]}) is None
+
+
 # --------------------------------------------------------------------------- #
 # Move to environment variable (issue #83)
 # --------------------------------------------------------------------------- #