feat: add quick install script and packaging (#197)

Give bot-bottle a real distribution path so new users can install
without cloning the repo:

- pyproject.toml: full project metadata, a `bot-bottle` console-script
  entry point (bot_bottle.cli:main), and package-data for the runtime
  assets (Dockerfiles, egress entrypoint, netpool defaults, macos init).
  Still zero runtime pip dependencies.
- install.sh: POSIX, sudo-free, idempotent bootstrapper — checks Python
  >= 3.11, creates ~/.bot-bottle/{agents,bottles,contrib}, installs via
  pipx (pip --user fallback), then runs `bot-bottle doctor`.
- `bot-bottle doctor`: new store-free subcommand reporting Python
  version, backend availability (reuses is_backend_available rather than
  hardcoding Docker), and config-dir presence. Exits non-zero when a hard
  prerequisite is unmet.
- PRD prd-new-install-script and unit tests for doctor, the packaging
  contract, and the install script.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/cli/commands/help.py

@@ -25,6 +25,7 @@ def cmd_help(argv: list[str] | None = None) -> int:
     w("  backend   set up / check / undo a backend's host prerequisites (setup|status|teardown)\n")
     w("  cleanup   stop and remove all active bot-bottle containers\n")
     w("  commit    snapshot a running bottle's container state to a Docker image\n")
+    w("  doctor    check host prerequisites (Python, backend, config dir)\n")
     w("  edit      open an agent in vim for editing\n")
     w("  help      show this command list\n")
     w("  init      interactively create a new agent and add it to bot-bottle.json\n")