[security] fix(sessions): validate session_id before deleting session files (#412)

* fix(sessions): validate session_id before deleting files

* fix: remove premature session index invalidation before validation check

* docs: v0.50.32 release — version badge and CHANGELOG

---------

Co-authored-by: hinotoi-agent <paperlantern.agent@gmail.com>
Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>

CHANGELOG.md

@@ -1,5 +1,14 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.32] fix(sessions): validate session_id before deleting session files [SECURITY] (#409)
+
+`/api/session/delete` accepted arbitrary `session_id` values from the request body and built the delete path directly as `SESSION_DIR / f"{sid}.json"`. Because pathlib discards the prefix when `sid` is an absolute path, an attacker could supply `/tmp/victim` and cause the server to unlink `victim.json` outside the session store. Traversal-style values (`../../etc/target`) were also accepted. CVSS 8.1 High (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H).
+
+- `api/routes.py`: validate `session_id` against `[0-9a-z_]+` allowlist (covers `uuid4().hex[:12]` WebUI IDs and `YYYYMMDD_HHMMSS_hex` CLI IDs) before path construction; resolve candidate path and enforce `path.relative_to(SESSION_DIR)` containment before unlinking; only invalidate session index on successful deletion path, not on rejected requests
+- `tests/test_sprint3.py`: 2 new regression tests — absolute-path payload rejected and file preserved, traversal payload rejected and file preserved
+- Original PR by @Hinotoi-agent (cherry-picked; branch was 4 commits behind master)
+- 1041 tests total (up from 1039)
+
 ## [v0.50.31] fix: delegate all live model fetching to agent's provider_model_ids()
 
 `_handle_live_models()` in `api/routes.py` previously maintained its own per-provider fetch logic and returned `not_supported` for Anthropic, Google, and Gemini. Now it delegates entirely to the agent's `hermes_cli.models.provider_model_ids()` — the single authoritative resolver — and `_fetchLiveModels()` in `ui.js` no longer skips any provider.