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/schemas.ts

@@ -178,6 +178,7 @@ export const QuickStartSchema = z.object({
     .optional(),
   mode: z.enum(['claude', 'shell', 'opencode']).optional(),
   openCodeConfig: OpenCodeConfigSchema,
+  envOverrides: safeEnvOverridesSchema,
 });
 
 // ========== Hook Events ==========
@@ -416,6 +417,7 @@ export const FlickerFilterSchema = z.object({
 export const QuickRunSchema = z.object({
   prompt: z.string().min(1).max(100000),
   workingDir: safePathSchema.optional(),
+  envOverrides: safeEnvOverridesSchema,
 });
 
 /** POST /api/scheduled */
@@ -538,6 +540,7 @@ export const RalphLoopStartSchema = z.object({
   completionPhrase: z.string().max(100).default('COMPLETE'),
   maxIterations: z.number().int().min(0).max(1000).nullable().default(10),
   enableRespawn: z.boolean().default(false),
+  envOverrides: safeEnvOverridesSchema,
   planItems: z
     .array(
       z.object({