docs: cross-platform focus-aware notification example (WSL + macOS + Linux)

[davissekai]

Issue for this PR

Closes #4846
Related #213

Type of change

• Documentation

What does this PR do?

The plugins docs showed a macOS-only osascript example for notifications. It fails on WSL/Linux and most community plugins are also macOS-only.

This updates packages/web/src/content/docs/plugins.mdx to a cross-platform, focus-aware example:

• WSL via PowerShell Windows.UI.Notifications toast
• macOS via osascript
• Linux via notify-send
• Only pings when the terminal is not frontmost (checks GetForegroundWindow on WSL, osascript frontmost on macOS, xdotool on Linux)
• Handles session.idle, session.error, and permission.updated / permission.ask with debounce, visual only by default

It also points to the community plugin opencode-notify-crossx (https://github.com/davissekai/opencode-notify-crossx) for the full source, and shows a single-file local alternative that works without npm.

How did you verify your code works?

• Restarted opencode on WSL Ubuntu 22.04, log shows notify plugin loaded (focus-aware)
• While opencode was focused, permission ask was suppressed: suppressed (focused) at 16:04:50
• Switched to Chrome, same permission triggered toast: opencode — input needed at 16:05:52
• Also verified session.idle suppress/notify in the same way

Screenshots / recordings

Tested via Windows toast — no screenshot needed for docs-only change. Log lines above show both paths.

Checklist

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

[davissekai]

Fixed PR description to follow template — added Issue, Type of change, What does this PR do, How verified, Screenshots, Checklist sections. Ready for review.

[github-actions]

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