#10186: fix: clear model/provider override when /new starts a fresh session

## Summary - When using `/new` to start a fresh session, the model override from the previous session was carried over - Root cause: the skills-snapshot persistence path in `agent.ts` uses `...current` to spread the previous `SessionEntry`, which includes `modelOverride` and `providerOverride` - Fix: explicitly set `modelOverride` and `providerOverride` to `undefined` when `isNewSession` is true - Repro: set default model to MiniMax → switch to Opus via `/opus` → send `/new` → session stays on Opus instead of reverting to MiniMax Closes #10107 ## Test plan - [x] Verified the spread operator at line 207 (`...current`) copies all fields from the old session entry - [x] The new conditional spread clears overrides only when `isNewSession` is true, leaving normal session updates unaffected - [x] Existing session fields (skillsSnapshot, sessionId, updatedAt) are preserved correctly 🤖 Generated with [Claude Code](https://claude.ai/code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates `src/commands/agent.ts` so that when `/new` starts a fresh session, any persisted `modelOverride` / `providerOverride` from the prior session are explicitly cleared while persisting the new session’s `skillsSnapshot`. This prevents model/provider selections made via `/opus`-style commands from leaking into the next “fresh” session, restoring the configured defaults. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is narrowly scoped to session persistence on new-session creation and uses the existing session store write path; no behavioral changes occur for non-new sessions. Clearing overrides via explicit `undefined` prevents them from being re-selected on subsequent model resolution. - src/commands/agent.ts <!-- greptile_other_comments_section --> **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->