feat(setup): add Claude Desktop MCPB bundle

[gaduviera]

Summary

Adds engram setup claude-desktop, which generates a local MCPB bundle for Claude Desktop regular chats and Cowork spaces.

The generated bundle:

• writes to ~/.engram/mcpb/engram-claude-desktop.mcpb
• launches the resolved Engram binary as an MCP stdio server
• uses the safer agent profile: engram mcp --tools=agent
• avoids editing claude_desktop_config.json directly

This gives Claude Desktop users a one-click install path similar to the Claude Code/OpenCode setup flows, while keeping the richer lifecycle hooks scoped to Claude Code.

Changes

• Add claude-desktop to supported setup agents
• Generate a minimal MCPB zip with manifest.json and README
• Add CLI post-install guidance for installing the .mcpb in Claude Desktop
• Document Claude Desktop/Cowork setup in README, Agent Setup, Plugins, and Architecture docs
• Add tests for supported-agent discovery and MCPB bundle contents

Validation

Passed locally on Windows:

go test ./internal/setup -run 'TestSupportedAgentsIncludesGeminiCodexAndClaudeDesktop|TestInstallClaudeDesktopWritesMCPBBundle' -count=1
go test ./cmd/engram -run '^$' -count=1
go build -o .tmp/engram-test.exe ./cmd/engram
.tmp/engram-test.exe setup claude-desktop
npx -y @anthropic-ai/mcpb info ~/.engram/mcpb/engram-claude-desktop.mcpb

mcpb info recognizes the generated bundle and reports only the expected unsigned warning for a local/private bundle.

Note: the full go test ./internal/setup suite currently has unrelated Windows-environment failures in this checkout (sh missing and existing Gemini/Codex/Claude Code fixture expectations). The new targeted tests pass.

[Alan-TheGentleman]

Thanks for exploring Claude Desktop MCPB support. I am closing this PR because setup and packaging surface needs an approved design issue before implementation. Open an issue that explains the intended Claude Desktop contract, what files Engram owns, and how this fits beside the existing setup flows. Once that is approved, a focused PR is welcome.

[gaduviera]

Thanks for the guidance. I opened #501 to define the Claude Desktop/Cowork MCPB contract, Engram-owned files, setup boundaries, upgrade behavior, and how it fits beside the existing Claude Code and agent setup flows. I'll wait for design approval there before preparing a new focused implementation PR.