feat: add multi-provider agents (claude/codex/gemini) with robust tracking and UI

[Akasha53]

Hello, this is my first time doing a Pull Request and I hope it works well. When i tried it works perfectly but as we say you never know so I would be very happy if you can give me feedback and if you find it great.

Summary

• Implement multi-provider architecture for Claude, Codex, and Gemini using provider definitions.
• Add provider detection, recommended default selection, saved provider preference, and backward-compatible openClaude alias.
• Generalize agent lifecycle (launch/restore/persist/remove) to include provider/session metadata and mixed-provider agents in one workspace.
• Add provider-aware session file binding for Codex and Gemini with resilient fallback matching when CLIs reuse existing sessions.
• Extend transcript parsing for provider-specific formats:
  • Claude: existing tool/sub-agent/permission tracking.
  • Codex: function_call/function_call_output + task_complete handling.
  • Gemini: session JSON snapshot parsing + text-turn thinking activity support.
• Harden file watching/read path to avoid empty-session-path errors.
• Add first-run provider behavior improvements when multiple CLIs are installed.

Webview/UI

• Replace openClaude flow with openAgent while preserving compatibility.
• Add provider state handling in extension messages.
• Add provider quick picker next to + Agent and provider controls in Settings.
• Include provider metadata in existing/restored agents and show correct behavior for per-agent launches.
• Add compatibility handling for legacy providersDetected payload shape.

Docs

• Update README and CHANGELOG with multi-provider support and migration notes.

Result

• Mixed provider sessions work in one panel.
• Existing Claude-only behavior remains compatible.
• Improved resilience for Codex/Gemini binding and activity display in real-world local session formats.