feat: add support for codex and antigravity agents

[romanxmanuel]

Description

Added support for multiple AI coding agents: Claude, Codex, and Antigravity.

• UI Updates: Replaced the hard-coded Open Claude button with an Agent selection dropdown menu. Handled workspace folder prompts gracefully.
• Extension Logic Updates: Refactored �gentManager to accept an AgentType parameter. Terminals are now dynamically created with specific commands based on the agent type (e.g. \codex --session-id).
• Activity Tracking: Standardized log monitoring to include ~/.codex/projects\ and ~/.antigravity/projects\ paths, allowing all three agents' JSONL logs to trigger active tool/typing animations within the Pixel Agents webview.