perf: reduce project loading requests

[winterfx]

Summary

• load project summaries first and fetch full project specs only after selecting a project
• add a lightweight UI-server project/agent context endpoint backed by bounded concurrent GetProject(includeSpec=false) calls
• reuse the lightweight context in Automations, Sandboxes, and the command palette
• avoid the full-project scan when previewing a new automation
• build MCP/Skills resources and targets from a single project-spec traversal

No changes to the agent-compose backend API are required.

Testing

• npm run check:ui
• go test ./...
• npx prettier --check src/api/agents.ts src/api/loaders.ts src/api/runs.ts src/routes/Automations.svelte src/components/AutomationCollection.svelte src/lib/components/command-palette.svelte
• git diff --check
• verified /api/ui/v1/project-agent-context against a local agent-compose daemon