feat(opencode): add lsp_min_severity config option

[KKdev15]

Issue for this PR

Closes #17869

Type of change

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

What does this PR do?

Adds a min_severity config option per-LSP server to control the minimum diagnostic severity level shown to the AI agent.

Currently, only Error (severity 1) diagnostics are displayed. Many LSP servers report style violations and linting issues as Warnings (severity 2), which are filtered out. This prevents the AI from seeing and fixing these issues.

With this change, users can configure each LSP to show warnings (or info/hints) by adding min_severity to their LSP config:

{
  "lsp": {
    "markdownlint": {
      "command": ["markdownlint-lsp"],
      "extensions": [".md"],
      "min_severity": 2
    }
  }
}

Severity levels: 1=Error (default), 2=Warning, 3=Info, 4=Hint.

Disclaimer: I would like to contribute this feature, as it's something I would really like to have! This MR was created only using AI. I'm not a TypeScript developer, so please be gentle on me :)

How did you verify your code works?

• Typecheck passes (bun typecheck)
• All existing tests pass (1332 pass, 1 unrelated flaky test failure)
• Added new test file test/lsp/min-severity.test.ts with 10 tests covering:
  • Config schema accepts min_severity per-LSP
  • Config schema validates range (1-4, integers only)
  • LSP.Diagnostic.filter() correctly filters by severity level
• Locally built opencode and integrated markdownlint-lsp server with the new option min_severity set to 4. Afterwards ask opencode to remove empty lines after headings and show the LSP output with severity. Here's a screenshot of the output, which verifies, that the warning for no empty line around headings (MD022) is actually reported as warning and visible to opencode:
  • to reproduce you have to disable formatter for opencode in your opencode.json, because the formatter automatically adds the newline again... ("formatter": false)

Screenshots / recordings

N/A - no UI changes

Checklist

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

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