Req: Document plan mode in README

[kevcube]

I saw the tweet about the new plan mode, I see it in the TUI, but there isn't good documentation.

Just a blurb in the readme is enough to satisfy me

[RawToast]

Maybe there should be something in the readme, but there's documentation for modes on the site:

https://opencode.ai/docs/modes/

[jayair]

Recently this got merged into agents, and the plan doc is here: https://opencode.ai/docs/agents/#plan

[dhofheinz]

Online docs are complete as noted. Added brief Agents section to README per @RawToast's suggestion (#3047).

Highlights plan agent's read-only mode (edit: deny, bash: ask) for safe codebase exploration. Kept minimal per README style - links to https://opencode.ai/docs/agents for full details.