feat(apiproxy): host session-log download surface

Streams one ZIP of the root session artifact plus each subagent descendant
verbatim (the persistence readRaw bytes) from GET /api/session.export as a
host-only download — no wire envelope, absent from IApiClient. The downloads
domain owns the query schema, the fetch handler answers the GET alongside the
SSE routes, and compression runs on the host with fflate's streaming Zip API.

THIRD_PARTY_NOTICES.md

@@ -59,6 +59,7 @@ External packages that a workspace package resolves at runtime. `scripts/install
 | [`diff`](https://github.com/kpdecker/jsdiff) | BSD-3-Clause |
 | [`e2b`](https://github.com/e2b-dev/e2b) | MIT |
 | [`eventsource-parser`](https://github.com/rexxars/eventsource-parser) | MIT |
+| [`fflate`](https://github.com/101arrowz/fflate) | MIT |
 | [`handlebars`](https://github.com/handlebars-lang/handlebars.js) | MIT |
 | [`immer`](https://github.com/immerjs/immer) | MIT |
 | [`js-yaml`](https://github.com/nodeca/js-yaml) | MIT |