Installer shouldn't auto-install AI agent hooks (and the opt-out is undocumented)

Running the standard install script (`curl -fsSL https://basecamp.com/install-cli | bash`) automatically configures agents like Claude and Codex. This means it installs their plugins and hooks globally without asking.

I just wanted the Basecamp CLI, so having Codex hooks suddenly injected into my global environment felt pretty intrusive.

Two main issues here:

1. **There is no documented way to opt out.** The README mentions it connects detected agents, but doesn't tell users they can use `BASECAMP_SETUP_AGENT=none` to skip it. I had to dig into the source code to find that.
2. **It should be opt-in.** Auto-installing background hooks without explicit consent is a big assumption to make about a user's environment.

**Suggestion:**
Could you update the installer so agent integration requires an explicit opt-in (like a prompt during the interactive setup)? At the very least, please add a note in the README so people know how to skip the agent installation.

Start with the standard install script and README, then trace how BASECAMP_SETUP_AGENT=none is handled for detected agents such as Claude and Codex. Confirm the installer asks before configuring agent plugins and hooks, and document the opt-out in the README.

Written by the indexing model from the issue text.