Refresh stale AGENTS.md (styling architecture, events, interactivity, deps)

[joestump-agent]

Part of #54 (found while researching it). AGENTS.md describes an architecture several PRs old and will actively mislead anyone working on styling:

• AGENTS.md:53 — "Styles live in render/styles.go as package-level lipgloss.Style vars" — false since feat: accept host theme/style set via WithStyles option #31 (Styles struct + DefaultStyles() + WithStyles); the options pattern isn't mentioned at all.
• AGENTS.md:26 — "Only ButtonClicked is actually emitted today" — false (InputSubmitted, ChoiceSelected ship).
• AGENTS.md:49 — "Only buttons are interactive" — false.
• AGENTS.md:70 — "Button.Action is not yet wired" — false (render/render.go:398-448).
• AGENTS.md:71 — "The data model (updateDataModel) is not applied" — false (render/composite.go:119-128).
• AGENTS.md:72 — "Input components are read-only" — false.
• AGENTS.md:73 — "First tab is always active. Modal renders only its trigger" — false (tabs Tabs: interactive tab switching (only the first tab renders today) #45, modals).
• AGENTS.md:65 — references render/deps.go, which no longer exists.
• The action package exists but is never mentioned.

Also: website/docs/intro.md:17 still says "the main remaining gap is interactive tab switching" — that landed via #45/PR #50.

Refresh both, and update the styling section again when #56/#57 land (or fold this into that series).