feat: NVIDIA thinking mode as opt-in toggle — full tool access by default for Kimi K2.5 / DeepSeek R1

[Copilot]

Kimi K2.5 and DeepSeek R1 were hardcoded to always use chat_template_kwargs: {thinking: true}, which causes NVIDIA NIM to reject any tools array (HTTP 400) — permanently disabling grep, bash, read, write, and all other agent tools for these models. Other IDEs (opencode, etc.) use the same API with full tool calling because they don't force thinking mode. This PR makes thinking mode opt-in and adds a live toggle in the chat UI.

Core behavior change (v2/src/core/agent-loop.mjs)

supportsThinking is now gated on an env var instead of model name alone:

// Before: thinking always on → tools always off
const supportsThinking = NVIDIA_THINKING_MODELS.has(model);

// After: opt-in via env var; default is full tool-calling
const thinkingEnabled = process.env.NVIDIA_THINKING_MODE === 'true';
const supportsThinking = thinkingEnabled && NVIDIA_THINKING_CAPABLE_MODELS.has(model);

In-panel toggle (media/chat.html / chat.js / chat.css)

• 💭 Thinking label + pill toggle appended to the controls bar, only visible when Kimi K2.5 or DeepSeek R1 is selected
• Toggle turns accent-colored when active for clear visual feedback
• On change, sends { type: 'thinkingMode', enabled } to the extension; bridge restarts automatically so the new env var is picked up
• State restored from saved config on panel open/reload via initialized message

Extension wiring (extension.js)

• New thinkingMode message handler: persists nvidiaThinkingMode to workspace config and disposes the bridge
• initialized payload now includes thinkingMode boolean
• NVIDIA_THINKING_MODE env var passed to bridge subprocess from config value

Mode comparison

Mode	Tools	Thinking trace
Default (toggle off)	✅ All 25+ tools	❌
Thinking mode (toggle on)	❌ workspace snapshot injected	✅