F6: kimi-cli command parity — kigi acp, mcp auth, --mcp-config-file

- `kigi acp`: top-level alias for the stdio ACP server (kimi-cli `acp`).
  AgentArgs.mode is now optional — bare `kigi agent` and `kigi acp` both
  default to stdio at dispatch; `kigi acp <mode>` is rejected.
- `kigi mcp auth <name>`: authorize an OAuth-enabled remote MCP server
  (kimi-cli `mcp auth`). Reuses the doctor's interactive connection path:
  starts the named server with OauthInteractivity::Interactive (browser
  flow when required), completes the handshake, and reports the tool
  count. Stdio servers and unknown names fail with actionable errors.
- `--mcp-config-file <PATH>` (repeatable, global): extra MCP config files
  in the .mcp.json shape ({"mcpServers": {...}}), kimi-cli semantics.
  Files are validated at parse time (fail fast on unreadable/invalid
  JSON), carried across the TUI -> shell boundary via
  KIGI_MCP_CONFIG_FILES, and merged at HIGHEST priority — an explicitly
  passed file overrides every config scope. Covered by loader unit tests
  and verified live: an injected server surfaces in the session's
  x.ai/mcp/servers_updated notification via both the flag and the env.

The stale bare-agent-requires-mode CLI test is re-contracted to the new
default-to-stdio behavior.

crates/codegen/kigi-tui/src/app/mod.rs

@@ -1359,12 +1359,15 @@ mod tests {
         assert!(args.no_leader);
         assert!(matches!(args.command, Some(Command::Agent(_))));
     }
-    /// The `agent` subcommand requires an explicit mode (stdio|serve|leader);
-    /// the old bare `agent` headless form was removed.
+    /// Bare `agent` parses with no mode and defaults to stdio at dispatch
+    /// (kimi-cli parity, F6 — the `acp` alias relies on the same default).
     #[test]
-    fn cli_bare_agent_subcommand_requires_mode() {
-        assert!(try_parse_pager(&["grok-pager", "--leader", "agent"]).is_err());
-        assert!(try_parse_pager(&["grok-pager", "agent"]).is_err());
+    fn cli_bare_agent_subcommand_defaults_to_stdio() {
+        let args = try_parse_pager(&["grok-pager", "agent"]).unwrap();
+        let Some(Command::Agent(agent)) = args.command else {
+            panic!("expected agent subcommand");
+        };
+        assert!(agent.mode.is_none(), "bare agent carries no explicit mode");
     }
     #[test]
     fn leader_defaults_off_without_config() {