fix(gjc): resume native sessions from their owning store

[devswha]

Summary

• keep new ChatMux GJC runs in the isolated scratch session store
• resume scratch-owned sessions from that store
• let native GJC TUI sessions fall back to GJC's default session lookup
• omit ChatMux's default picker sentinel instead of passing it as a GJC model id

Root causes

ChatMux always passed --session-dir /tmp/gjc-live-sessions to gjc -r. Native TUI sessions are stored under GJC's normal home session tree, so history resume could not find them. After correcting lookup, the resume still failed because the UI fallback value default was emitted as --model default; it is a sentinel, not an installed GJC model.

Verification

• node --import tsx --test server/gjc-cli.test.ts (21/21)
• direct native-session resume with GJC 0.11.6
• npm run typecheck
• npm run lint
• npm run build

[devswha]

E2E QA evidence (2026-07-22): after this fix, the actual browser history-resume turn completed with CHATMUX_E2E_GJC_T2 value=GJC_T1. Targeted GJC tests and the final integration verify gate passed.