feat: add /uptime slash command

[mateuszkwiatkowski]

Issue for this PR

Closes #24160

Type of change

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

What does this PR do?

Adds a /uptime slash command that shows how long the OpenCode process has been running, similar to the UNIX uptime command. It opens a centered dialog (same style as /status) displaying:

• Current time and process uptime, e.g. 11:23 AM up 3h 1m 45s
• Session idle time (duration since last completed assistant message), e.g. idle 5m 12s
• If the session is actively streaming, shows working instead
• If on the home screen (no active session), the idle line is omitted

Changes:

• opencode-process.ts: records Date.now() at module load as the process start time
• /global/health endpoint: now includes startTime in the response
• dialog-uptime.tsx: new dialog component matching the /status dialog layout, with idle time tracking via route and sync context
• app.tsx: registers the /uptime slash command under the System category
• SDK types regenerated to include startTime field

How did you verify your code works?

• Ran /uptime in the TUI from both session and home views
• Confirmed idle duration updates based on last assistant message completion
• Confirmed working state shown during active streaming
• bun typecheck passes (all 13 packages via turbo pre-push hook)
• SDK regenerated via ./packages/sdk/js/script/build.ts
• Linter (oxlint) reports no new warnings

Screenshots / recordings

N/A -- no structural UI change beyond a new dialog following the existing /status pattern.

Checklist

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

[rektide]

I'd love to also show how long we are idle for, as well

[mateuszkwiatkowski]

@rektide Opencode already has idle/working status concept in sync.session.status(). This PR is kind of related: #15738 but I don't think these two are in conflict so I'll add it for you.