fix(opencode): add null checks for agent.current() in prompt

[tdawe1]

Fixes #7918

What does this PR do?

Adds null safety checks for local.agent.current() across the TUI codebase.
local.agent.current() can return undefined when:

• Auth plugins (like opencode-antigravity-auth) trigger before agent initialization
• The agents array is empty during startup
  This caused TypeError: undefined is not an object crashes.
  Files changed:
• prompt/index.tsx: 6 fixes
• local.tsx: 8 fixes
• dialog-agent.tsx: 1 fix
  All use optional chaining (?.) with fallback to "default".

How did you verify your code works?

1. Built locally with ./packages/opencode/script/build.ts --single
2. Tested with opencode-antigravity-auth plugin enabled
3. Confirmed TUI no longer crashes during auth flow
4. Pre-push typecheck passes

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate PRs Found

I found 2 related PRs that may be addressing the same issue:

1. PR fix(tui): handle undefined agent.current() to prevent fatal TypeError #7689 - fix(tui): handle undefined agent.current() to prevent fatal TypeError

   • fix(tui): handle undefined agent.current() to prevent fatal TypeError #7689
   • Directly addresses the same TypeError issue with undefined agent.current()

2. PR fix(tui): handle undefined agent.current() to prevent crash on startup #7748 - fix(tui): handle undefined agent.current() to prevent crash on startup

   • fix(tui): handle undefined agent.current() to prevent crash on startup #7748
   • Focuses on startup crashes related to undefined agent.current()

Both of these PRs appear to be tackling the same null safety issue for agent.current() that PR #7920 is addressing. You may want to check if these are already merged or closed, and coordinate with any concurrent work.