Add CODEOWNERS to auto-request reviews on touched paths

[turbomam]

No CODEOWNERS file exists. Adding one would auto-request reviews from the right person when a PR touches their area, and (combined with branch protection's "require code-owner reviews") can enforce domain-expert approval.

Suggested starting structure

# Default owner for everything
*                             @turbomam

# Ontology editing / templates
/src/ontology/                @turbomam
/src/templates/               @turbomam

# Python package
/metpo/                       @turbomam

# Literature mining
/literature_mining/           @turbomam

# CI / workflows
/.github/                     @turbomam

Replace/expand with additional owners as collaborators take on specific areas.

File lives at .github/CODEOWNERS, CODEOWNERS, or docs/CODEOWNERS. Branch protection on main already exists but does not require code-owner review yet — that toggle can be flipped once CODEOWNERS is populated.

Discovered during repo cleanup 2026-04-17.