feat: parse GitHub.copilot-chat/transcripts/*.jsonl event-stream format

Summary

VS Code with the GitHub Copilot Chat extension writes session data in two distinct locations:

1. workspaceStorage/<hash>/GitHub.copilot-chat/ — the existing format already parsed by the extension (per-session JSON blobs).
2. GitHub.copilot-chat/transcripts/*.jsonl — a newer event-stream format written at the user-data level, alongside the workspace storage directory.

The extension currently only reads format 1. Format 2 is silently ignored, so sessions recorded there never appear in the dashboard.

The transcript format

Each .jsonl file is a newline-delimited stream of typed events:

Who is affected

• WSL / VS Code Server / Remote SSH / Dev Container users are most likely to encounter this because their workspaceStorage path is nested under ~/.vscode-server/ (fixed in #63), but the transcripts/ directory sits at ~/.vscode-server/data/User/GitHub.copilot-chat/transcripts/.
• Desktop users writing to the standard user-data directory are equally affected if their sessions happen to be stored in the newer format.

Proposed solution

Add two functions to parser-vscode.ts:

• listTranscriptFiles(dir: string): string[] — finds all *.jsonl files under a transcripts/ subfolder.
• parseTranscriptFile(filePath: string): SessionData | null — reads the event stream and returns a SessionData object (same shape the rest of the pipeline expects), grouping events into turns and deduplicating tool names.

Wire both into the existing processWorkspaceEntry / processWorkspaceEntryAsync functions so the dashboard picks up sessions from either format transparently.

Acceptance criteria

• parseTranscriptFile correctly maps event-stream turns to SessionData.requests[]
• Tool names are deduplicated per turn
• Empty/malformed files return null gracefully
• Existing tests remain green
• New unit tests cover: multi-turn grouping, empty session, tool deduplication, and full flow

Start in parser-vscode.ts and trace processWorkspaceEntry and processWorkspaceEntryAsync to understand how existing workspace sessions become SessionData. Add transcript discovery and event-stream parsing there, then verify multi-turn grouping, empty or malformed files, per-turn tool deduplication, and the full flow with new unit tests while keeping existing tests green.