Update AgentLive.Index to properly handle browser back/forward buttons
by syncing the setup token modal to URL parameters.
Changes:
- Update handle_params/3 to read modal param and set modal state
- Update show_setup handler to use push_patch with ?modal=setup
- Update close_token_modal handler to use push_patch (remove modal param)
- Update handle_agent_creation_success to use push_patch after creating agent
Behavior:
- Browser back button now closes the setup modal without navigating away
- URL reflects current modal state (?modal=setup)
- Bookmarkable URLs restore exact modal state (can share agent setup URL)
- After creating agent, modal opens via URL state
Follows idiomatic Phoenix LiveView pattern using push_patch/2 and
handle_params/3 as documented in CLAUDE.md.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>