Rename /demo playground route to /playground; print the actual URL on startup

[AmitAvital1]

Current behavior

Running the local UI stack:

docker run -p 8100:8100 -v "$(pwd):/workspace" -w /workspace \
  --env-file .env \
  docker.io/library/extra:local agent-manager --config agents.yml

only prints uvicorn's generic startup line:

INFO:     Uvicorn running on http://0.0.0.0:8100

This is misleading — the root path isn't the app; the actual chat playground UI lives at /demo (src/agent_manager/api/web.py:32). Nothing in the CLI output tells the user that. Separately, the path /demo itself isn't a great name for what's effectively a UI playground for testing agents.

Steps to reproduce

1. make up-ui (or the docker command above)
2. Observe the only printed line is the generic uvicorn one, pointing at 0.0.0.0:8100
3. Have to already know (from docs/Makefile) to go to http://localhost:8100/demo to see anything

Expected behavior

1. On startup, agent-manager prints an explicit, clickable line pointing at the actual playground, e.g.:

   Playground: http://localhost:8100/playground
   

   (in src/agent_manager/cli.py, after create_app/before or after uvicorn.run)
2. The route itself is renamed from /demo to /playground (src/agent_manager/api/web.py), with every reference updated:
   • Makefile (up-ui target + its @echo line)
   • tests/agent_manager/test_web.py
   • docs mentioning /demo: README.md, docs/WIDGET.md, docs/WIDGET_ARCHITECTURE.md, docs/ROADMAP.md, docs/MCP_AND_TOOLS.md, docs/runtime-hooks.mdx
   • (out of scope: /widget-demo*.html routes are a separate widget-embedding demo, not the playground — left as-is unless we decide otherwise)