Onboard to Overwatch: add overwatch.yml

[catio-overwatch-integration]

Adds a draft overwatch.yml so this repo becomes a graded Overwatch citizen
(closes meta.overwatch_yml_present).

Opened by the Overwatch Generate overwatch.yml workflow as part of the onboarding
rollout. Before merging, please confirm a few fields:

• owner — set the owning @org/team (or a list of people) if the guess is wrong.
• kind — pre-filled from a best-effort guess; correct it if wrong.
• deploy.strategy — guessed from the repo's branches; confirm trunk/gitflow.
• classification.tier — defaulted to 1 (production). Adjust down to 2/3/4
  if this isn't a production system.

Full schema: the Overwatch /docs page.