feat!: rewrite all internals to use use acp

[cloud-on-prem]

This is a wholesale rewrite.

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Replaces HTTP-based server architecture with direct ACP (Agent Communication Protocol) subprocess communication via JSON-RPC 2.0 over stdin/stdout.

Key changes:

• New JsonRpcClient for ndjson-framed JSON-RPC 2.0 communication
• SubprocessManager for goose binary lifecycle (spawn, graceful shutdown, version gating ≥1.16.0)
• SessionManager with fp-ts TaskEither for session create/load with history replay
• Consolidated src/webview/ React UI (removes separate webview-ui/ package and Vite tooling)
• Unified webpack build for extension + webview
• Shared types in src/shared/ for type-safe message passing

Removed:

• HTTP server (src/server/)
• ApiClient, ServerManager, ChatProcessor
• Separate webview package with Vite config

Closes #27
Closes #25