chore: add ownership-guard action

[flaticols]

Adds flaticols/ownership-guard@v2.0.0 to enforce cross-team ownership awareness on PRs.

What this does

• Adds .ownership file defining the core team with ownership over internal/** and cmd/**
• Adds .github/workflows/ownership.yml that runs on every PR

Testing

This draft PR itself is the test — the ownership workflow should fire and post a warning comment if the PR author is not in the core team.

.ownership syntax

team: core
@flaticols/core

=ownership:
internal/**
cmd/**

[github-actions]

Important

internal is part of the analysis engine owned by core. @flaticols, please coordinate with @flaticols/core — changes here affect the core refactoring logic.

[github-actions]

Note

internal/tui is owned by the tui team. @flaticols, loop in @flaticols/core — UI changes should be reviewed for keyboard, rendering, and accessibility consistency.

[github-actions]

Warning

nix is owned by infra. @flaticols, please align with @flaticols/core before merging — nix changes affect the binary packaging and release pipeline.