fix(tui): handle null agent gracefully in TUI

[jwcrystal]

Issue for this PR

Closes #16982

Type of change

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

What does this PR do?

This fixes the TUI crash when local.agent.current() is read while the visible agent list is empty or when the stored current agent is no longer valid.

Compared with #17015, this version also fixes the state management side of the bug:

• agentStore.current is now string | undefined
• initialization uses agents()[0]?.name instead of assuming an agent always exists
• agent.current() returns undefined only when the visible agent list is empty
• if the list is non-empty but the stored current agent is stale, it falls back to the first visible agent
• a sync createEffect keeps agentStore.current valid when the available agent list changes
• move() now handles an empty agent list safely

It also updates the TUI call sites to handle the missing-agent case gracefully:

• submit now exits early with a warning toast when no agent is available
• dialog and prompt UI use optional chaining / fallback values instead of assuming .name always exists
• fallback label is now No agent

How did you verify your code works?

• Ran bun typecheck in packages/opencode
• Verified the modified files compile cleanly after changing agent.current() to nullable behavior
• Reviewed all touched agent.current() call sites to ensure they now handle the missing-agent case safely

Screenshots / recordings

Not included. This is a null-safety fix for a crash path rather than a visual UI change.

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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

Based on the search results, I found a highly relevant related PR:

Potentially Related PR

PR #17015 - "Fix/issue 16982 agent current undefined"

• Fix/issue 16982 agent current undefined #17015
• This PR appears to address the same core issue: handling when agent.current() returns undefined. It may have already tackled some aspects of this problem, so you should verify whether this PR fully resolves the issues or if PR fix(tui): handle null agent gracefully in TUI #18868 addresses additional edge cases (OAuth failure, runtime removal, startup).

Additionally, these PRs have related themes around null safety in the agent system:

• PR fix: remove unsafe non-null assertions in agent.ts #18701 - "fix: remove unsafe non-null assertions in agent.ts"
• PR fix: add null checks for LSP status and event type validation #18703 - "fix: add null checks for LSP status and event type validation"

These suggest an ongoing effort to improve null safety in the codebase. Check if PR #17015 is already merged and whether PR #18868 extends its fix to cover TUI-specific scenarios.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[rekram1-node]

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