refactor: pass envOverrides via tmux export instead of disk write

CLAUDE_CODE_EFFORT_LEVEL (and any CLAUDE_CODE_* / OPENCODE_* key) now flows:
  UI dropdown → POST /api/sessions { envOverrides }
             → new Session({ envOverrides })
             → this._envOverrides
             → tmux-manager.buildEnvExports appends `export KEY=<shellescape(VALUE)>`

Previously the API wrote envOverrides to <case>/.claude/settings.local.json, which
created stale state (UI dropdown disagreeing with disk) and polluted user project
directories. Now envOverrides are ephemeral spawn-time state, preserved across
respawnPane cycles via this._envOverrides and across server restart via
SessionState.envOverrides in state.json.

Also removes the now-unused updateCaseEnvVars import from session-routes.ts.

src/web/public/ralph-wizard.js

@@ -1032,6 +1032,10 @@ Object.assign(CodemanApp.prototype, {
     const enabledItems = config.generatedPlan?.filter(i => i.enabled);
 
     try {
+      const envOverrides = this.buildEnvOverrides(
+        this.getCaseSettings(config.caseName),
+        this.loadAppSettingsFromStorage()
+      );
       const res = await fetch('/api/ralph-loop/start', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
@@ -1042,6 +1046,7 @@ Object.assign(CodemanApp.prototype, {
           maxIterations: config.maxIterations || null,
           enableRespawn: config.enableRespawn,
           planItems: enabledItems?.length ? enabledItems : undefined,
+          ...(Object.keys(envOverrides).length > 0 ? { envOverrides } : {}),
         }),
       });
       const data = await res.json();