feat(deploy-keys): expose Deploy Keys management (Free)

[polaz]

👍 React to this issue if you need this feature — helps us prioritise.

Problem

Deploy Keys are how CI systems push to repos without using user PATs. GitLab has full CRUD for them but we expose nothing — agents can't audit/rotate deploy keys.

Investigation done (current state)

Verified against src/entities/:

• ❌ grep -ri 'deploy.*key\|deploy_key' src/ returns 0 hits

Re-verify before coding:

grep -rIni 'deploy.*key' src/entities/

Acceptance criteria

• New entity src/entities/deploy_keys/ with:
  • browse_deploy_keys actions: list (instance / project), get
  • manage_deploy_key actions: add, enable (existing key from another project), update (title, can_push), delete
• Instance-level list requires admin (uses feat(context): probe admin_mode elevation and expose in whoami #434 admin probe)
• Integration test creating SSH keypair, adding to test project, enabling, deleting

GitLab API

• Endpoints: /api/v4/deploy_keys, /projects/:id/deploy_keys, /projects/:id/deploy_keys/:key_id/enable
• Tier: Free
• Docs: https://docs.gitlab.com/api/deploy_keys/

Estimate

4h

Context

Found during GitLab 18→19 API landscape analysis.