feat(opencode): surface lsp diagnostics on read

[mmsaki]

Issue for this PR

Closes #16569

Type of change

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

What does this PR do?

Adds a diagnostics operation to the built-in lsp tool and updates read to surface current-file LSP diagnostics when a language server is available.
The lsp tool now accepts file-level diagnostics without requiring line/character, while position-based operations still require them. The read tool now attaches warning/error diagnostics for the current file as a best-effort enhancement and keeps normal read behavior if diagnostics lookup fails.

How did you verify your code works?

• Ran bun test test/tool/read.test.ts test/tool/lsp.test.ts in packages/opencode
• Ran bun run typecheck in packages/opencode
• Manually tested the built binary to confirm:
  • read surfaces diagnostics for supported file types
  • lsp diagnostics works without positional arguments

Screenshots / recordings

openode-diagnostics.mp4

Checklist

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

[mmsaki]

@thdxr is this a valid contribution see demo?