feat: implement comprehensive exec mode for cortex-cli (#191)

This implements a complete headless execution mode similar to Droid's exec
mode from Factory. The new exec command is designed for CI/CD pipelines,
shell scripts, and batch processing.

Key features:
- Autonomy levels (--auto): read-only, low, medium, high
  - read-only: No file/tool permissions
  - low: Requires approval for writes
  - medium: Auto-approve safe operations
  - high: Full auto-approve
- Skip permissions (--skip-permissions-unsafe): Bypass sandboxing
- Output formats (--format):
  - text: Human-readable streaming output (default)
  - json: Structured JSON final result
  - stream-json: NDJSON event stream
  - stream-jsonrpc: JSON-RPC 2.0 protocol for multi-turn
- Input formats (--input-format):
  - text: Single prompt input
  - stream-jsonrpc: Multi-turn JSON-RPC input via stdin
- Tool controls:
  - --enabled-tools: Whitelist specific tools
  - --disabled-tools: Blacklist specific tools
- Session continuation: --session-id for resuming sessions
- Execution limits:
  - --timeout: Maximum execution time
  - --max-turns: Maximum conversation turns
- Fail-fast behavior: Aborts on permission policy violations
- Image attachments: --image flag for visual inputs

JSON-RPC 2.0 support for multi-turn conversations:
- Submit user messages via stdin
- Receive events via stdout
- Supports cancel, status, and heartbeat operations
- Configurable heartbeat interval

This replaces the simple ExecCommand with ExecCli providing
enterprise-grade automation capabilities.

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt