fix(core): verify existing umbrella feature branches

[lafawnduh1966]

Summary

Implements BRIX-1535.

• Adds an umbrella_feature_branch_missing guard for existing umbrella workflows.
• Creates umbrella feature branches only for brand-new workflows; existing workflows now verify the persisted branch exists instead of recreating it from base.
• Applies the guard to Linear parent re-enqueue, umbrella subtask enqueue, and final umbrella PR reconciliation before PR/task side effects.
• Documents the new troubleshooting path.

Validation

• bun test packages/cli/tests/cli/enqueue_linear_issue.test.ts
• bun test packages/cli/tests/pr/umbrella_final_pr_reconcile.test.ts packages/cli/tests/enqueue/umbrella_enqueue.test.ts packages/cli/tests/core/umbrella_workflows.test.ts
• bun test packages/cli/tests/cli/enqueue_linear_issue.test.ts packages/cli/tests/pr/umbrella_final_pr_reconcile.test.ts packages/cli/tests/enqueue/umbrella_enqueue.test.ts packages/cli/tests/core/umbrella_workflows.test.ts
• bun run scripts/embed.ts from packages/cli
• bun run typecheck:cli

[linear]

BRIX-1535

[lafawnduh1966]

lgtm!