invoke lint should assert the package layout instead of the loader failing on it #83

Context. _load_extension in tasks/schemas.py runs infrahubctl schema load <path> on
any directory it is handed. A directory under extensions/ or experimental/ that
contains no .yml makes that call fail with an infrahubctl error rather than a message
naming the repo rule it broke. docs/docs/contributing.mdx:11-13 states the rule: a
directory, one <schema_name>.yml, and a .metadata.yml entry.

This PR removed extensions/meet_me_room, the only directory that violated the rule, so
the marker now describes a hypothetical rather than a live failure. It is worth keeping as
a check, not as a comment.

Current marker.

tasks/schemas.py:16
    # TODO: here if in extensions folder we have a dir without schema it will fail

What this asks for. Add a structure assertion to invoke lint: every directory under
extensions/ and experimental/ has exactly one .yml and exactly one matching
.metadata.yml key, and every .metadata.yml key has a directory. Fail the lint task
with the offending path. Out of scope for #75, which does not own the tooling.

Origin: pass-1 review of #75, packet STRUCT-13, CC-8 bucket (c).