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/mux-interface.ts

@@ -63,6 +63,8 @@ export interface CreateSessionOptions {
   openCodeConfig?: OpenCodeConfig;
   /** When restoring after reboot, resume a previous Claude conversation by its session ID */
   resumeSessionId?: string;
+  /** Extra env vars exported before launching the CLI (e.g., CLAUDE_CODE_EFFORT_LEVEL). Ephemeral — not written to disk. */
+  envOverrides?: Record<string, string>;
 }
 
 /** Options for respawning a dead pane. */
@@ -77,6 +79,8 @@ export interface RespawnPaneOptions {
   openCodeConfig?: OpenCodeConfig;
   /** Resume a previous Claude conversation when respawning */
   resumeSessionId?: string;
+  /** Extra env vars exported before launching the CLI (preserved across respawns). */
+  envOverrides?: Record<string, string>;
 }
 
 /**