Add AGENTS.md agent overview

[alltheseas]

• Overview: outlines how Notedeck’s agent-style apps fit into the repo, emphasizing architecture, coding patterns, and shared services.
  • Repo map: describes key crates (core host, chrome shell, columns client, shared UI, Dave agent, token utilities) and their responsibilities.
  • Core abstractions: details the App trait, Notedeck host, AppContext, and AppResponse flow that power app integration.
  • UI/navigation: explains chrome container behavior, app registration via NotedeckApp, preview/theming support, and shared egui components.
  • Data/networking: covers NostrDB usage, caching layers, relay pool subscriptions, and debounced persistence utilities.
  • Async patterns: highlights job pool usage, streaming updates (from Dave), and relay event handling.
  • Conventions: stresses strict linting, modular structure, tracing/profiling, localization workflow, and testing practices.
  • Agent integration checklist: step-by-step guidance for building, registering, theming, localizing, and wiring new agents.
  • Reference links: points to developer docs across crates for deeper dives into architecture, UI components, and agent tooling.

[Copilot]

Pull Request Overview

Adds AGENTS.md to document how agent-style apps integrate with Notedeck, covering architecture, shared services, UI patterns, data/networking, async patterns, conventions, and an integration checklist.

• Introduces a high-level map of crates and responsibilities
• Documents core abstractions (App, Notedeck host, AppContext, AppResponse) and UI/container patterns
• Provides an agent integration checklist and references to deeper docs

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[Copilot]

[nitpick] Terminology mixes 'Columns' and 'Damus' in different places. To avoid confusion, standardize the naming (e.g., 'Damus (notedeck_columns)') here and use the same term consistently throughout the doc.