[Feature Request] Allow agents to send initial message on startup

Bug Description
Feature request: Allow agents (--agent) to auto-send an initial message on
startup.

Currently, custom agents always wait for user input before responding.
There's no way to make an agent proactively display a menu or greeting
when the session starts.

Proposed: Add an "initialMessage" field in agent definition
(.claude/agents/*.md)
that the agent sends automatically on session start, without requiring user
input.

Use case: Interactive agents with menus (e.g., a vault management agent
that
shows 6 action options on startup).

Environment Info

• Platform: darwin
• Terminal: xterm-256color
• Version: 2.1.92
• Feedback ID: e6152809-23de-4182-babc-2ca37ef7165d

Errors

[{"error":"Error: Failed to fetch version from https://storage.googleapis.com/claude-code-dist-86c565f3-f756-42ad-8dfa-d59b1c096819/claude-code-releases/latest: Request failed with status code 429\n    at Py1 (/$bunfs/root/src/entrypoints/cli.js:1882:15046)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-04T00:51:13.550Z"},{"error":"Error: Failed to fetch version from https://storage.googleapis.com/claude-code-dist-86c565f3-f756-42ad-8dfa-d59b1c096819/claude-code-releases/latest: Request failed with status code 429\n    at Py1 (/$bunfs/root/src/entrypoints/cli.js:1882:15046)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-04T00:51:13.551Z"},{"error":"Error: Failed to fetch version from https://storage.googleapis.com/claude-code-dist-86c565f3-f756-42ad-8dfa-d59b1c096819/claude-code-releases/latest: Request failed with status code 429\n    at Py1 (/$bunfs/root/src/entrypoints/cli.js:1882:15046)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-04T00:56:15.733Z"},{"error":"Error: Failed to fetch version from https://storage.googleapis.com/claude-code-dist-86c565f3-f756-42ad-8dfa-d59b1c096819/claude-code-releases/latest: Request failed with status code 429\n    at Py1 (/$bunfs/root/src/entrypoints/cli.js:1882:15046)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-04T00:56:15.733Z"},{"error":"Error: console.error: [bun] Warning: ws.WebSocket 'unexpected-response' event is not implemented in bun\n    at K (/$bunfs/root/src/entrypoints/cli.js:528:10765)\n    at emitWarning (ws:43:42)\n    at #onOrOnce (ws:170:18)\n    at AU_ (/$bunfs/root/src/entrypoints/cli.js:3190:4680)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-04T01:12:01.494Z"},{"error":"Error: Request was aborted.\n    at makeRequest (/$bunfs/root/src/entrypoints/cli.js:50:3447)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-04T01:27:07.611Z"},{"error":"Error: Request was aborted.\n    at makeRequest (/$bunfs/root/src/entrypoints/cli.js:50:3447)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-04T01:28:57.128Z"}]