Headless mode: support per-invocation system prompt append and MCP config

What feature would you like to see?

We are integrating Kimi Code as an external headless agent runtime. kimi -p --output-format stream-json is already a good fit for this use case:

• one invocation has a clear turn boundary;
• print mode uses automatic permissions;
• stream-json is machine-readable;
• --skills-dir already works;
• the final session.resume_hint exposes a session id that can be passed back through --session <id>.

Two per-invocation host inputs are still missing:

1. Append-only host system instructions

   Allow a host to append instructions to Kimi's normal effective system prompt without replacing the native profile and without writing an AGENTS.md file.

2. Explicit MCP configuration for this invocation

   Allow a host to provide the MCP servers for a -p invocation, with an opt-in replace mode that ignores MCP servers discovered from the user's, project's, or plugin configuration.

The exact option names are only illustrative:

kimi -p "..." \
  --output-format stream-json \
  --append-system-prompt-file /path/to/host-role.md \
  --mcp-config /path/to/mcp.json \
  --mcp-config-mode replace

Expected semantics:

• The appended prompt is applied after Kimi's normal profile/project instructions for both new and --session <id> resumed invocations.
• It is scoped to the invocation, is not persisted as project/user configuration, and does not write into the workspace or Kimi home.
• In MCP replace mode, the supplied file is the complete external MCP server set; an empty set means no external MCP servers.
• Without these options, current behavior is unchanged.

This keeps host-owned role instructions separate from repository instructions and lets a host supply dynamic, session-specific MCP endpoints while continuing to use the user's normal Kimi home, login, model configuration, and session storage.

Additional information

Current main already forwards --skills-dir into print mode's harness, so no skill-directory change is requested here:

• run-prompt.ts
• machine-readable session.resume_hint

The underlying v1 session creation/resume path already has caller-supplied MCP inputs, but it merges them with discovered native MCP configuration:

• new session MCP resolution
• resumed session MCP resolution

The public print CLI currently does not expose either the host prompt append or the caller MCP input.

Start with the print-mode forwarding in apps/kimi-code/src/cli/run-prompt.ts and prompt handling in apps/kimi-code/src/cli/prompt-render.ts. Then trace the new and resumed session MCP resolution in packages/agent-core/src/rpc/core-impl.ts. Done means per-invocation prompt append and MCP configuration work for new and resumed print sessions, including replace mode, without changing behavior when the options are absent.