refactor(cli): simplify agent mode validation flow

What I changed:
- extracted a single missingModeError helper in agent/mode.ts so the command no longer duplicates the same command error payload in two branches
- flattened control flow by returning early for --list and using one set-mode path guarded by explicit missing-mode validation
- tightened tests/10-agent-mode.test.ts to assert the exact missing-mode error message instead of allowing a daemon-connection fallback

Reasoning:
- this keeps behavior the same while reducing branch duplication and making the validation contract explicit
- the hardened test protects the intended precedence: missing required mode should fail before any daemon interaction

Verification:
- npm run typecheck
- (cd packages/cli && npx tsx tests/10-agent-mode.test.ts)

Notes for next agent:
- repository had a pre-existing local modification in scripts/codex-refactor-loop.sh; I intentionally did not touch or stage it
- no blockers encountered for this refactor

packages/cli/tests/10-agent-mode.test.ts

@@ -125,12 +125,10 @@ try {
     // Should fail because mode is required unless --list is specified
     assert.notStrictEqual(result.exitCode, 0, 'should fail without mode argument')
     const output = result.stdout + result.stderr
-    const hasError =
-      output.toLowerCase().includes('missing') ||
-      output.toLowerCase().includes('required') ||
-      output.toLowerCase().includes('mode') ||
-      output.toLowerCase().includes('daemon') // If daemon error comes first, that's also valid
-    assert(hasError, 'error should mention missing mode or connection issue')
+    assert(
+      output.includes('Mode argument required unless --list is specified'),
+      'error should mention missing mode argument'
+    )
     console.log('✓ agent mode requires mode argument when not using --list\n')
   }
 } finally {