fix(tui): force terminal reset on exit for Windows ConPTY

[kraso]

Issue for this PR

Closes #48776

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

On Windows ConPTY stacks (Alacritty + zellij), exiting opencode left the terminal
in a raw/corrupted state and, in some setups, terminated the parent shell/pane.

Two things caused it:

• @opentui/core's native renderer teardown closes the shared console handle on
  Windows, which kills the parent shell / zellij pane. The OS releases that
  handle when the process exits, so the call is skipped on win32.
• The exit reset emitted \x1b[?1049l (leave alternate screen), which races with
  process teardown on ConPTY and can also kill the pane. It is replaced with
  \x1b[2J\x1b[H (clear screen + home) instead.

The reset now also runs synchronously via fs.writeSync on every exit path
(/exit, SIGHUP, SIGTERM, and process.on("exit")).

How did you verify your code works?

• bun typecheck in packages/tui — no new errors (baseline and patched trees
  report identical counts).
• bun run lint (oxlint) — no new findings.
• Built opencode-windows-x64 with the repo-pinned bun (script/build.ts --single),
  smoke test (--version) passed.
• Manual test on Windows with Alacritty + zellij (0.43.1): consecutive
  open//exit cycles leave zellij intact and the screen clean, and the session
  epilogue still prints.

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Recording from the earlier repro/discussion on the same stack:
https://github.com/user-attachments/assets/8c574259-de03-4b0c-a6db-089e671562dc

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR