docs: design for browser-only agents via just-bash CLI mode + WASM

[Copilot]

Adds a detailed design document for running mcp-compressor just-bash CLI mode entirely in a browser (except LLM vendor calls), covering how to bridge the Rust-native / napi-rs dependency gap via WebAssembly.

What's added

• docs/browser-agent-design.md — full design covering:
  • mcp-compressor-wasm crate: wasm-bindgen wrapper around the pure-computation half of the Rust core (compression, cli/parser, cli/help, ffi/pure) behind a pure-only Cargo feature that strips Tokio/axum/reqwest/rmcp
  • BrowserMcpClient: TypeScript fetch-based MCP Streamable HTTP transport replacing the Rust session manager
  • Just-bash browser integration: existing createJustBashCommandRegistrations / installJustBashRegistrations are already pure TS, work unchanged once WASM provides the pure functions
  • OAuth PKCE helper replacing the loopback callback server
  • Optional Service Worker pattern for persistent cross-tab sessions
  • Browser package entry point (./browser export) wiring the WASM adapter behind the same NativeCore interface
  • Tradeoffs, constraints table, and a 3-phase implementation roadmap
  • 6 mermaid diagrams (overall architecture, fetch sequence, agent command lifecycle, OAuth PKCE flow, today-vs-new-work breakdown, Gantt roadmap)
• mkdocs.yml — nav entry under Development

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