docs: append Feb 23 mod system operational summary

SESSION_LOG.md

@@ -87,8 +87,42 @@ Chronological ledger of notable execution work. Keep this high signal; link to c
 
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+## 2026-02-23 — Mod System Now Operational
+
+Full mod lifecycle implemented and confirmed end-to-end.
+
+### What shipped
+- Modrinth search integrated (`GET /api/game/mods/search`)
+- Full install/list/enable/disable/delete routes live behind auth + ownership
+- Agent host allowlist expanded to include `cdn.modrinth.com`
+- Filename validation updated to allow `+` (required for Modrinth filenames like `sodium-neoforge-0.6.13+mc1.21.4.jar`)
+- Engine metadata bug fixed: `engineType="neoforge"`, `engineVersion="1.21.4"` (was `"minecraft"` / `"neoforge-1.21.4"`)
+- API → Agent payload contract corrected (field names and `source` field)
+- Frontend: mod search drawer, installed panel, enable/disable/delete, toast notifications
+
+### System state
+- Full lifecycle via API: ✅
+- Frontend search + install: ✅
+- Enable/disable/delete: ✅
+- Installed flag displayed: ✅
+- Soft delete to `/mods-removed`: ✅
+- No retention automation: intentional
+- No install queue: intentional
+- Filesystem authoritative: confirmed
+
+### Known tradeoffs locked in
+- No deterministic Modrinth project ID persistence (installed detection is heuristic)
+- No DB tracking of mod state
+- Soft delete is permanent until file browser enables manual restore
+
+### Pending
+- Response corruption investigation (one early curl output appeared malformed)
+- API error mapping: `"mod already exists"` → `409` (currently `502`)
+- Portal UI wiring for install flow
+
+---
+
 ## Next session targets
-- API: proxy agent mod + process metrics endpoints behind auth/ownership.
-- Artifacts: add vanilla 1.21.8–1.21.11 jars + confirm NeoForge naming contract resolved.
-- Frontend: add Mods UI surface using API proxy endpoints.
-- zlh-artifactd: Phase 1 design (Modrinth resolver, no local mod storage).
+- Reproduce and resolve response corruption with clean curl test
+- Tighten API error mapping
+- Begin file browser (agent + API + frontend) — see `OPEN_THREADS.md`