fix(cli): reject legacy umbrella config on Linear enqueue

[lafawnduh1966]

Summary

Implements BRIX-1534.

• Rejects any parsed quay-config umbrella: metadata during quay enqueue --linear-issue before validator, Linear relation reads, hierarchy reads, or substrate side effects.
• Removes the remaining Linear-backed fallback that could pass legacy umbrella config into core enqueue.
• Updates ticket authoring docs to state that Linear-backed tickets must use native Linear parent/child and blocked-by relations instead of umbrella config metadata.

Validation

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

[linear]

BRIX-1534

[lafawnduh1966]

lgtm!