fix: inherit blocked status through parent hierarchy (Refs: beans-8mhc)

[teal-bauer]

Summary

Approach 1 for #92: children inherit their parent's blocked status.

• FindActiveBlockers now walks up the parent chain after checking direct blockers
• If any ancestor has active blockers, they're included in the result
• IsBlocked returns true transitively — no API changes needed
• Cycle-safe (visited set + depth cap)

Simple and opinionated: if your parent is blocked, you're blocked.

Trade-offs

• Can't prep child tasks while a parent epic is blocked at the epic level
• No way to distinguish "directly blocked" from "blocked via ancestor"

See #92 for design discussion and the alternative approach in #94.

Test plan

• go test ./... passes
• Child of blocked parent → blocked
• Grandchild of blocked grandparent → blocked
• Child of unblocked parent → not blocked
• Resolved parent blocker → not blocked
• Child with own blocker + blocked parent → both blockers returned