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_core.py

@@ -2051,6 +2051,54 @@ def args_secret_warning(data: dict) -> str | None:
     return None
 
 
+def secret_arg_indices(args: list) -> list[int]:
+    """Indices of args that look like a raw credential.
+
+    Same detection as `args_secret_warning`, but per-arg so the "move to
+    environment variable" action (#83) knows exactly which arg to offer on.
+    Flags a token-prefixed positional (ghp_..., sk-...), an embedded-credential
+    URL, or the value following a secret-named flag (`--token abc`). A `${VAR}`
+    reference is never flagged -- it's the fix, not the problem.
+    """
+    out: list[int] = []
+    mask_next = False
+    for i, a in enumerate(str(x) for x in args):
+        if is_env_ref(a):
+            mask_next = False
+            continue
+        if mask_next:
+            mask_next = False
+            if not a.startswith("-"):
+                out.append(i)
+            continue
+        if a.startswith("-") and "=" in a:
+            continue
+        if a.startswith("-") and is_secret_key(a):
+            mask_next = True
+            continue
+        if a.startswith("-"):
+            continue
+        if _is_secret_value(a) or _EMBEDDED_CRED_RE.search(a):
+            out.append(i)
+    return out
+
+
+def suggested_env_var_for_arg(args: list, index: int) -> str:
+    """A default variable name for moving `args[index]` to a reference.
+
+    Uses the preceding flag when there is one (`--api-key <secret>` ->
+    API_KEY), since that names what the value is; otherwise falls back to a
+    generic SECRET. Always a legal shell name.
+    """
+    if 0 < index <= len(args):
+        prev = str(args[index - 1]) if index - 1 < len(args) else ""
+        if prev.startswith("-"):
+            base = prev.lstrip("-").split("=", 1)[0]
+            if base:
+                return sanitize_env_var_name(base)
+    return "SECRET"
+
+
 def split_suspicious_args(args: list[str]) -> tuple[list[str], list[str]]:
     """
     Detect the classic argument-entry mistake: several argv tokens typed on one