docs: add Antigravity Context Meter to ecosystem & community tools

[Dunphil692]

Summary

This pull request adds Antigravity Context Meter to the community extensions and tools section.

Problem Solved

During deep, multi-turn reasoning and complex autonomous coding workflows in Antigravity, context saturation is a major factor that can lead to memory decay or session interruption. Querying context usage interactively via chat introduces unwanted prompt overhead and token costs.

Key Capabilities of Antigravity Context Meter

• Zero-Token Local Telemetry: Employs an ultra-fast local event-driven tokenizer (0.039ms latency) that streams real-time token metrics directly from transcripts without consuming any model API quota.
• Detailed Component Breakdown: Visualizes usage across System Rules, MCP Tool Schemas, User/Assistant Messages, and Large Tool Outputs with real-time health-level indicators.
• Multi-Project & Multi-Session Tracking: Supports seamless switching and pinning across concurrent Antigravity workspaces and desktop sessions.
• Zero-Loss Session Migration: Features automated, structured prompt distillation at 80% capacity to extract key architectural decisions and unfinished tasks, enabling seamless continuation in new sessions.
• Dual-Mode Ergonomics: Offers a lightweight native macOS floating HUD capsule (53 KB) alongside an embedded IDE status bar extension (.vsix).

Checklist

• Link is valid and points to the actively maintained open-source repository.
• Strictly focused on enhancing the Antigravity developer workflow.
• Adheres to project formatting guidelines and Google Open Source code of conduct.

[google-cla]

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[Dunphil692]

@google-cla I have signed the CLA!