feat(cortex-cli): implement complete headless exec mode

[echobt]

Summary

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

Features

Autonomy Levels (--auto)

• read-only: No file/tool permissions - agent cannot make changes
• low: Requires explicit approval for all write operations
• medium: Auto-approve safe operations (reads), prompt for writes
• high: Full auto-approve for all operations

Skip Permissions (--skip-permissions-unsafe)

Bypass all sandboxing for trusted environments (use with caution)

Output Formats (--format)

• text: Human-readable streaming output (default)
• json: Structured JSON final result
• stream-json: NDJSON event stream for real-time monitoring
• stream-jsonrpc: JSON-RPC 2.0 protocol for multi-turn conversations
• debug: Verbose debug output

Input Formats (--input-format)

• text: Single prompt input (default)
• stream-jsonrpc: Multi-turn JSON-RPC input via stdin

Tool Controls

• --enabled-tools: Whitelist specific tools (comma-separated)
• --disabled-tools: Blacklist specific tools (comma-separated)

Session Management

• --session-id: Continue an existing session

Execution Limits

• --timeout: Maximum execution time in seconds
• --max-turns: Maximum conversation turns

Other Features

• Fail-fast behavior: Aborts on permission policy violations
• Image attachments: --image flag for visual inputs
• JSON-RPC 2.0 support: Multi-turn conversations with stdin/stdout

JSON-RPC 2.0 Protocol

For multi-turn conversations:

• Submit user messages via stdin
• Receive events via stdout
• Supports cancel, status, and heartbeat operations
• Configurable heartbeat interval (--heartbeat)

Example Usage

# Simple single-shot execution
cortex exec "Help me fix the tests" --auto high

# Read-only mode for safe exploration  
cortex exec "Analyze this codebase" --auto read-only

# JSON output for scripting
cortex exec "List all TODOs" --format json

# Multi-turn conversation mode
cortex exec --input-format stream-jsonrpc --format stream-jsonrpc

# With tool restrictions
cortex exec "Write some code" --auto high --disabled-tools Execute

# With timeout and turn limits
cortex exec "Fix bugs" --auto high --timeout 300 --max-turns 50

Migration Notes

This PR replaces the simple ExecCommand with the comprehensive ExecCli from the new exec_cmd module. The basic usage remains compatible, but additional flags are now available for fine-grained control.

Testing

• Code compiles successfully
• cargo fmt applied
• All existing tests pass

Related Issues

• Implements headless exec mode similar to Factory's Droid