Add Observer local agent canvas and coordination plugin

[NaveDanan]

Adds Observer as an external plugin, pinned to the tested source commit. It shows local coding-agent sessions, tool calls, and agent relationships on a browser canvas, and includes six employee agents plus a stdio MCP server for peer coordination.

Installation requires Node.js 22.5+ and npm. After installing, users ask "Set up Observer for this computer". The setup script downloads the existing v0.9.21 GitHub release, verifies its SHA-256, and installs the runtime in ~/.observer/runtimes/0.9.21. Hook execution never downloads software. Before setup, telemetry hooks exit quietly and MCP reports the missing runtime.

Session data stays under ~/.observer; the daemon listens on loopback. Setup contacts GitHub and npm. Users should keep only one Observer hook/plugin installation enabled for each host.

Validation on Windows:

• Installed the plugin through a clean Copilot configuration and a repository marketplace.
• Tested every generated Windows telemetry command with a plugin path containing spaces.
• Verified checksum download, npm installation, daemon startup, MCP initialization, and a synthetic session reaching the local API.
• Passed portable-launch and catalog consistency tests (node --test scripts/test-marketplace.mjs).

Installation and data-handling details. This PR requests catalog inclusion; it does not claim existing GitHub endorsement or approval.