feat(#83): offer move-to-env on args rows; explain the gate instead of an empty menu

Two things surfaced testing the GUI:

1. Args rows showed the secret warning but no move action -- the args
   editor is a free-text widget, not a table, and was deliberately left out
   of the first cut. Wired it up: ArgsEdit gains a context menu that offers
   "Move to environment variable…" on exactly the args that look like a
   credential. New pure core: secret_arg_indices (which args are secrets,
   mirroring args_secret_warning per-index) and suggested_env_var_for_arg
   (default var name from the preceding flag -- `--api-key <secret>` ->
   API_KEY, else SECRET). The move replaces that one arg line with ${VAR}
   and copies the secret to the clipboard, same contract as the tables.

2. On a Claude Desktop profile (or any non-expanding client) the menu showed
   NOTHING, so it read as broken. Now a real stored secret always shows the
   item -- enabled on a client that expands references, or disabled with the
   reason ("unavailable for Claude Desktop -- it doesn't expand ${VAR}") so
   the gate is visible rather than silent. Applies to env, headers and args.

Not a change: after converting, env_ref_warnings still notes a variable that
isn't set in the environment. That's #82's advisory doing its job -- the user
runs the export line the dialog handed them; auto-adding a ':-default' would
bake a fallback back into the config and defeat moving the secret out.

Tests: +5 core (secret_arg_indices for token/flag-value/embedded-URL/
reference-excluded, suggested_env_var_for_arg with and without a flag).
483 passed, ruff clean. GUI wiring (context menus) remains untestable in CI.

Refs #83

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EKwBecy6N83jnqQmw8ezwE

bcc.py

@@ -418,12 +418,23 @@ class KeyValueTable(QWidget):
             return
         row = item.row()
         key, value = self._row_key_value(row)
-        profile = self.profile_provider() if self.profile_provider else None
-        if not core.can_move_value_to_env_ref(key, value, profile):
+        # Only a real stored secret is worth moving; nothing to offer otherwise.
+        if not core.should_mask_value(key, value):
             return
+        profile = self.profile_provider() if self.profile_provider else None
         menu = QMenu(self)
         act = QAction("Move to environment variable…", self)
-        act.triggered.connect(lambda: self._move_row_to_env(row))
+        if core.can_move_value_to_env_ref(key, value, profile):
+            act.triggered.connect(lambda: self._move_row_to_env(row))
+        else:
+            # Show it disabled with the reason rather than an empty menu, so the
+            # feature is discoverable and Claude Desktop's gating is explained.
+            act.setEnabled(False)
+            act.setText("Move to environment variable — unavailable for Claude Desktop")
+            act.setToolTip(
+                "Claude Desktop doesn't expand ${VAR}, so a reference would reach "
+                "the server as literal text."
+            )
         menu.addAction(act)
         menu.exec(self.table.viewport().mapToGlobal(pos))
 
@@ -762,10 +773,11 @@ class ServerEditor(QFrame):
         return w
 
     def set_profile_provider(self, provider):
-        """Let the env/headers tables gate "move to environment variable" on
-        which client the loaded profile targets (#83)."""
+        """Let the env/headers tables and the args editor gate "move to
+        environment variable" on which client the loaded profile targets (#83)."""
         self.env.profile_provider = provider
         self.headers.profile_provider = provider
+        self.args.profile_provider = provider
 
     # --- model <-> form -------------------------------------------------- #
     def load_entry(self, entry: core.ServerEntry | None):
@@ -1044,6 +1056,50 @@ class ArgsEdit(QPlainTextEdit):
         self.blockCountChanged.connect(self._update_gutter_width)
         self.updateRequest.connect(self._on_update_request)
         self._update_gutter_width()
+        # Set by ServerEditor so "move to environment variable" (#83) can gate on
+        # the loaded client, same as the env/headers tables.
+        self.profile_provider = None
+
+    def contextMenuEvent(self, event):
+        menu = self.createStandardContextMenu()  # keep cut/copy/paste
+        lines = self.toPlainText().splitlines()
+        block = self.cursorForPosition(event.pos()).blockNumber()
+        if 0 <= block < len(lines) and lines[block].strip():
+            # This editor is one arg per line; map the clicked block to its
+            # index among the non-blank args the model actually sees.
+            cleaned = [ln for ln in lines if ln.strip() != ""]
+            idx = sum(1 for ln in lines[:block] if ln.strip() != "")
+            if idx in set(core.secret_arg_indices(cleaned)):
+                profile = self.profile_provider() if self.profile_provider else None
+                act = QAction("Move to environment variable…", self)
+                if profile is None or core.client_expands_env_refs(profile):
+                    act.triggered.connect(lambda: self._move_arg_to_env(block, idx, cleaned))
+                else:
+                    act.setEnabled(False)
+                    act.setText("Move to environment variable — unavailable for Claude Desktop")
+                    act.setToolTip(
+                        "Claude Desktop doesn't expand ${VAR}, so a reference would "
+                        "reach the server as literal text."
+                    )
+                first = menu.actions()[0] if menu.actions() else None
+                menu.insertAction(first, act)
+                if first is not None:
+                    menu.insertSeparator(first)
+        menu.exec(event.globalPos())
+
+    def _move_arg_to_env(self, block: int, idx: int, cleaned: list):
+        value = cleaned[idx]
+        default_name = core.suggested_env_var_for_arg(cleaned, idx)
+        dlg = MoveToEnvDialog(self.window(), default_name, value)
+        if not dlg.exec():
+            return
+        var_name = dlg.var_name()
+        QGuiApplication.clipboard().setText(value)
+        lines = self.toPlainText().splitlines()
+        if 0 <= block < len(lines):
+            lines[block] = f"${{{var_name}}}"
+            # setPlainText fires textChanged -> _emit (dirty + arg recheck).
+            self.setPlainText("\n".join(lines))
 
     def gutter_width(self) -> int:
         digits = max(1, len(str(self.blockCount())))