ci: migrate 3 of 3 jobs to smithy self-hosted runners

[avrabe]

Summary

Migrates the single CI workflow (checks.yml) on the org-wide
.github repo from GitHub-hosted to the pulseengine smithy fleet.
All three jobs are tiny validators (file-existence, markdown link
check, SVG XML well-formedness) that complete in seconds.

Coverage

Workflow	Job	Class	Notes
checks.yml	ci	light	test -f checks against profile/README.md and pipeline.svg
checks.yml	lint	light	grep-based markdown image-link validator
checks.yml	test	light	Python 3 xml.etree.ElementTree SVG parse

Stays on hosted

Job	Reason
(none)	All three jobs migrate cleanly

Workarounds applied

None. Python 3 is preinstalled on smithy, and grep / test are part
of any Linux base. No sudo, apt-get, container, or external
toolchain dependency.

Test plan

• All three jobs land on light runners on the next PR
• ci, lint, test all pass green on smithy
• Wall-clock improves (no GitHub-hosted queue)

Rollback

Revert this commit. The three runs-on: lines flip back to
ubuntu-latest.

References

• Migration playbook: https://github.com/pulseengine/smithy/blob/main/docs/migration-playbook.md
• Pilot PR: ci: pilot-migrate clippy job to smithy self-hosted runners spar#201