perf(continuity): stream first user prompt instead of parsing the whole transcript

[danikdanik]

Why

firstUserPromptFromSession resolved the first user message by calling loadMessagesFromSessionFile, which reads and JSON-parses every line of the session transcript. It only needs the first user message (which sits near the top of the file), and it runs on the cross-session continuity path that can fire on several early session:patch events before the first user prompt has landed on disk — so each of those calls re-parsed an ever-growing transcript.

What

Replace that call with a streaming reader (firstUserPromptFromSessionFile) that reads the file in 64 KB chunks and stops at the first non-empty user message:

• A StringDecoder buffers partial multi-byte UTF-8 sequences across chunk boundaries, so text is never corrupted mid-character.
• The line-level parse (userTextFromJsonlLine) mirrors loadMessagesFromSessionFile's content handling (plain string or array of text blocks), so behavior is unchanged.
• loadMessagesFromSessionFile remains imported — it is still used elsewhere.

How (verified)

Verified equivalent to the previous implementation across 6 cases: assistant-first ordering, array-block content, tool_result-only user turns, no-user files, a multi-byte char forced onto a chunk boundary, and a 5000-line file where it early-exits. tsc build is green; committed dist/ rebuilt.

[alexgreensh]

Merged in v5.11.13. Streaming the first prompt instead of parsing the whole transcript is a clean win, and the UTF-8 chunk-boundary handling checks out. Thanks!