tool-build: build MyGroomer (split + label raw issues)

[lorenzoliuzzo]

Part of the fleet milestone (#1 §2). Design: mythings-core/docs/tools/my-groomer.md.

MyGroomer splits/labels raw incoming issues into well-scoped, labeled work.

Dependency — now (nearly) unblocked

Needs create_issue / add_labels on the GitHub contract. These just landed
in mythings-core#33 — pick this up as soon as #33 merges. (list_labels may
still be wanted; confirm during build.)

Scope (v0)

• Input: a raw issue (label my-groomer).
• One Engine call: propose a split + labels.
• Output: child issues via create_issue + add_labels, linked to the parent.

Steps

• Confirm Bound headless session wall-clock time, harden account-usage probing, close CI test gap #33 merged (and whether list_labels is needed → file on core if so).
• Scaffold + implement + tests.