feat(terminal): add focus_on_open option

[zgs225]

Tasks

• I read [CONTRIBUTING.md](https://github.com/nickjvandyke/opencode.nvim/blob/main/
  CONTRIBUTING.md)
• I ensured my changes pass automated checks
• I reviewed and understand the AI-generated code in my changes (if any)
• I addressed actionable AI review feedback (if any)

Description

By default opencode.terminal.open() and toggle() return focus to the
previous window after opening. This adds a focus_on_open option that,
when true, keeps the cursor in the terminal and enters insert mode.
Affects three code paths: new terminal creation, re-showing a hidden
terminal, and the TermRequest redraw autocmd.

Fallback behavior is unchanged — omitting the option (or setting it
false/nil) preserves exactly the existing focus behavior.

Testing

1. Set up vim.g.opencode_opts with focus_on_open = true in both
   server.start and server.toggle, and map require("opencode").toggle().
2. Press the toggle keymap — verify the terminal opens with focus and
   is in insert mode (ready for input).
3. Press the toggle keymap again — verify the terminal hides.
4. Press the toggle keymap a third time — verify it re-shows with focus
   and insert mode.
5. Remove focus_on_open = true and repeat — verify focus returns to
   the previous window (original behavior unchanged).

Related Issue(s)

N/A

Screenshots/Videos

N/A

[nickjvandyke]

Thanks for the idea and effort! Unfortunately I intend to keep terminal.lua as a minimal and opinionated convenience (and workarounds for embedded opencode quirks). Other terminal plugins support extensive customization, so I don't see a need to re-implement that and take on the maintenance burden.

Sorry!!