core/cli: fix /help command usage line and help Tab completion

Use a standalone Click context so /help compact shows Usage: /compact,
not nested under help. Complete /help <cmd> from the slash registry.

src/plyngent/cli/readline_setup.py

@@ -60,17 +60,20 @@ def build_completer(state: ReplState) -> Callable[[str, int], str | None]:
 
 
 def _argument_options(state: ReplState, command: str, text: str) -> list[str]:
-    if command == "/provider":
-        return filter_prefix(text, sorted(state.config.providers.keys()))
-    if command == "/model":
-        return filter_prefix(text, sorted(state.provider.models.keys()))
-    if command in {"/tools", "/stream", "/verbose"}:
-        return filter_prefix(text, list(_ON_OFF_ARGS))
-    if command == "/export":
-        return filter_prefix(text, list(_EXPORT_ARGS))
-    if command == "/resume":
-        return []
-    return []
+    candidates: list[str] = []
+    if command == "/help":
+        # Second token: command name without leading slash (also accept /name).
+        candidates = [n.removeprefix("/") for n in slash_commands()]
+        text = text.lstrip("/")
+    elif command == "/provider":
+        candidates = sorted(state.config.providers.keys())
+    elif command == "/model":
+        candidates = sorted(state.provider.models.keys())
+    elif command in {"/tools", "/stream", "/verbose"}:
+        candidates = list(_ON_OFF_ARGS)
+    elif command == "/export":
+        candidates = list(_EXPORT_ARGS)
+    return filter_prefix(text, candidates)
 
 
 def bind_tab_complete(readline_mod: object) -> None:

src/plyngent/cli/slash.py

@@ -112,8 +112,8 @@ def help_cmd(ctx: click.Context, command: str | None) -> None:
         if cmd is None:
             click.echo(f"unknown command /{name}; try /help")
             return
-        # Build a child context so get_help includes usage.
-        with click.Context(cmd, info_name=name, parent=ctx) as sub:
+        # Standalone context (no parent=help) so usage is "/compact …" not "help … compact".
+        with click.Context(cmd, info_name=f"/{name}") as sub:
             click.echo(cmd.get_help(sub))
         return
     click.echo(slash.get_help(ctx))