Merged as v0.50.230. 2685 tests passing. Browser QA 21/21.
Closes the orphan-files leg of #1171. `new_session()` no longer writes an empty session to disk — the first disk write is deferred until the session has real state. Verified live: `POST /api/session/new` creates no `.json` file; session is findable by GET from in-memory SESSIONS dict.
Attribution: original PR #1184 by @nesquena (Claude Code).