feat(hooks): add turn_end observer hook

Harvested the narrow Rust/docs slice of PR #2578 by @AresNing for #1364. The event uses the maintained structured observer path: JSON stdin, stdout ignored, warn-only failures, and no ability to block or mutate the turn.

The hook fires after post-turn app state, usage totals, cost, notification, receipt, and queue-recovery state are updated, before queued follow-up dispatch. Docs, RFC notes, /hooks discovery, and v0.9 tracking now describe the observer-only contract.

Co-authored-by: AresNing <49557311+AresNing@users.noreply.github.com>

README.md

@@ -720,7 +720,7 @@ Current and recurring contributors include:
 - **[yuanchenglu](https://github.com/yuanchenglu)** — Feishu per-chat model switching (#2149)
 - **[HUQIANTAO](https://github.com/HUQIANTAO)** — Xiaomi balance/status work, stalled-turn recovery, approval intent summaries, mobile smoke/QR support, Claude theme, and broad docs/test/CI coverage (#2257, #2267, #2283, #2384, #2385, #2389, #2403, #2440-#2458, #2460)
 - **[h3c-hexin](https://github.com/h3c-hexin)** — web-search URL decoding, prompt/instructions override hooks, sub-agent guidance, SSRF fake-IP trust configuration, and prompt-cache-friendly environment placement (#2245, #2311, #2313, #2314, #2354, #2355, #2356)
-- **[AresNing](https://github.com/AresNing)** — first-run guide and message-submit hook transform design harvested into the maintained hooks path (#2278, #2318, #2434)
+- **[AresNing](https://github.com/AresNing)** — first-run guide, message-submit hook transform design, and turn-end observer hook work harvested into the maintained hooks path (#2278, #2318, #2434, #2578)
 - **[Implementist](https://github.com/Implementist)** — Volcengine Ark search provider and reliability hardening (#2426, #2429, #2439)
 - **[lihuan215](https://github.com/lihuan215)** — Unix socket hook sink design harvested into the opt-in hook event path (#2333, #2430)
 - **[AdityaVG13](https://github.com/AdityaVG13)** — Xiaomi MiMo provider support (#2246)