feat(cli): add --headless-interactive flag for multi-turn stdin sessions

[GoBeromsu]

Summary

• Add --headless-interactive CLI flag that enables headless interactive mode for programmatic consumers
• Reads newline-delimited prompts from stdin, maintains conversation state across turns, outputs to stdout
• No Ink UI required — works when stdin.isTTY === false

Context

Issue #13924 requests the ability to keep the CLI alive for multi-turn sessions when launched programmatically (e.g., from Java, Node.js, or backend services). Currently, isHeadlessMode() forces non-interactive mode when stdin is not a TTY, which reads all stdin as one prompt and exits.

I investigated existing solutions:

• ACP mode (--acp) technically provides headless interactive capabilities, but requires ndJSON protocol handshake (initialize → authenticate → newSession → prompt) and SDK dependency
• Non-interactive mode (--prompt) supports session resumption via --resume, but each invocation is independent — no persistent process
• PR feat(daemon): add stateful headless daemon mode #20700 addresses the related Feature Request: add a stateful headless mode (daemon/server mode) #15338 with a daemon mode (Unix sockets), which is a larger-scope solution

This PR offers a lightweight alternative: a plain-text stdin/stdout interface following the same pattern as nonInteractiveCli.ts. It's complementary to both ACP and the daemon approach — different interface for different use cases.

How to Validate

# Single prompt
echo "what is 2+2?" | gemini --headless-interactive

# Multi-turn session (second prompt references first)
node -e "
const { spawn } = require('child_process');
const child = spawn('npx', ['gemini', '--headless-interactive'], { stdio: ['pipe', 'pipe', 'inherit'] });
child.stdout.on('data', d => console.log(d.toString()));
child.stdin.write('My name is Alice.\n');
setTimeout(() => child.stdin.write('What is my name?\n'), 10000);
setTimeout(() => child.stdin.end(), 20000);
"

# With structured output
echo "hello" | gemini --headless-interactive --output-format stream-json

Changes

File	Change
packages/cli/src/config/config.ts	Add --headless-interactive flag, CliArgs field, mutual exclusivity checks
packages/cli/src/gemini.tsx	Add routing branch after ACP, before interactive mode
packages/cli/src/headlessInteractiveCli.ts	New readline-based headless REPL runner

Related Issues

Fixes #13924
Related: #15338, PR #20700

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • macOS
    • npm run

[GoBeromsu]

Closing draft — need to address review findings before resubmitting.