MIP-0086 task 15: MIP-0086 Implemented #786

What

Flip MIP-0086's status to Implemented, naming each task's PRs, in the MIP's metadata block and in its docs/MIPs/README.md row. Leave the Cost so far row as it is: the README says that row is no longer kept up since the Cost: trailer was dropped (marola-dev/marola-devkit#103, #794).

Every task row's PRs are merged. Row 13 (#740) waits only for its live auto-merge proof, which is #740's own acceptance criterion; row 15 does not depend on rows 13 or 14 (the tasks file's depends on column).

Acceptance criteria

• the MIP's Status row and its README row say Implemented and name each task's PRs
• mip_graph.py --check green

Named test

scripts/mip_graph.py --check.

MIP-0086 owns the design and its tasks file the order; this issue owns the status. Blockers are native blocked by edges from the tasks file's depends on column.