Rename host MCP to workspace MCP for clarity

- Rename binary from host-mcp to workspace-mcp
- Rename src/bin/host_mcp.rs to src/bin/workspace_mcp.rs
- Update tool prefix from host_* to workspace_*
- Update MCP registration name from "host" to "workspace"
- Add "Builtin" tag to UI for workspace and desktop MCPs
- Update documentation (CLAUDE.md, INSTALL.md, docs-site)

The "workspace MCP" name better reflects that it runs in the
workspace's execution context - inside containers for container
workspaces, on host for host workspaces.

Cargo.toml

@@ -79,8 +79,8 @@ name = "desktop-mcp"
 path = "src/bin/desktop_mcp.rs"
 
 [[bin]]
-name = "host-mcp"
-path = "src/bin/host_mcp.rs"
+name = "workspace-mcp"
+path = "src/bin/workspace_mcp.rs"
 
 [dev-dependencies]
 tokio-test = "0.4"