Add formatted JSONL viewer to diagnostics

[Copilot]

JSONL session files are newline-delimited JSON objects without array brackets or commas, making them difficult to read and inspect.

Changes

• Diagnostics UI: Added "Actions" column to session files table with "📄 View" link
• Extension method: showFormattedJsonlFile() reads JSONL, parses to JSON array, opens in untitled editor with syntax highlighting
• Message handler: openFormattedJsonlFile command bridges webview to extension

Implementation Details

Transforms this:

{"kind":0,"mode":{"id":"ask"},"model":"gpt-4"}
{"kind":1,"mode":{"id":"agent"}}
{"kind":2,"k":["requests"],"v":[{"requestId":"req1"}]}

Into formatted JSON array in untitled document:

[
  {
    "kind": 0,
    "mode": {
      "id": "ask"
    },
    "model": "gpt-4"
  },
  ...
]

• Non-destructive: original files never modified
• Type safe: uses unknown[] instead of any[]
• Error resilient: malformed lines skipped with line number warnings
• Auto-refresh: closes stale untitled documents before recreating

Files modified: src/extension.ts (+70 lines), src/webview/diagnostics/main.ts (+17 lines)

Original prompt

The jsonl files are very hard to read, as they are a streaming list of json objects, without the normal array brackets or commas at the end of the line.

I want to update the file viewer for this from the diagnostics view, without overwriting the file. Add the brackets and comma's, and then format the view.

Current contents:

{ object with fields } 
{ object with fields } 
{ object with fields } 
{ object with fields } 
{etc}

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