hub: subtasks and related cards — parent, related, children, policy.subtasks (§6.4, §6.1, §6.15)

tsk-3nf2kjev11m3 done feature Maity Coder ChatGPT 001 (maity-coder-chatgpt-001)

Contract merged in #346 (maintainer's inbox decision 2026-10-25). Implement docs/api.md §6.4 Subtasks in full:

Card fields parent (nullable task id), related (symmetric, mirrored like ships_with), computed children: {open, total}; GET …/tasks?parent=<id>; POST/PATCH accept parent/related under the table's rights.
Rules, all checked on the resulting card/graph in one project-locked transaction: two levels at most (too_deep, has_children, self, release_card), a closed parent takes no new child (parent_closed), cycle over the union of depends_on and the parent→child gate (cycle on the named field), done refused with dependencies_open while a child is open (details.open + details.children).
policy.subtasks (§6.1): proposed (default, projects that predate the field read as proposed) / direct; under proposed a caller without tasks:admin creates children as proposed, status: open in the body is forbidden subtasks_proposed, proposed→open on a child needs tasks:admin, attaching parent to a non-proposed card is forbidden for that caller — also when the same PATCH opens it.
Events: child task_updated fields [parent]; old and new parent fields [children] with counts, also on child close/cancel; related like ships_with. History (§6.15): parent, related; children never.
Migration 0048 (reserved for this card), parent = main's head at merge time — I ping the exact parent as for every migration in the queue; until then CI's migration check may be red.
Tests: every refusal code above, the bypass attempts named in the contract review (open-then-attach, attach-and-open in one PATCH, creator flipping proposed→open), the done gate with mixed depends_on/children, mirror events. help.md lines for the new fields and filter. Changelog fragment; context/ only if an implementation choice is worth it.

After #343 lands. Contract owner: A; one proposal to me on anything the contract leaves open.