chore: remove githubLinkId DEPRECATION suppressions after V9 migration

[jorisjonkers-dev-agents]

Follow-up from lint sweep (PR #10). 7 @Suppress("DEPRECATION") mark the githubLinkId migration window; remove them once the V9 migration drops the column. See CANDIDATES-agents-api.md.

[jorisjonkers-dev-agents]

Ready to pick up

Follow-up from the lint sweep (PR #10). Seven @Suppress("DEPRECATION") annotations guard githubLinkId usages during the migration window. These can be removed once a migration drops the github_link_id column from the workspaces table.

Current state (verified):

• V8__github_links.sql added workspaces.github_link_id UUID REFERENCES github_links(id).
• V9__repositories_and_session_kinds.sql introduced the repositories table (the replacement).
• V17__drop_deploy_keys.sql dropped deploy-key columns but did not drop github_link_id.
• No migration drops workspaces.github_link_id yet.
• 7 @Suppress("DEPRECATION") annotations remain (in Workspace.kt, CreateWorkspaceCommand.kt, CreateWorkspaceCommandHandler.kt, WorkspaceDtos.kt ×2, WorkspaceController.kt, JooqWorkspaceRepository.kt, and tests).

Work to do:

1. Add a migration (e.g. V22__drop_github_link_id.sql) that:
   • Migrates any remaining workspaces rows that still reference github_link_id to the repository_id equivalent (or nullifies them if no active workspaces use the legacy path).
   • Drops workspaces.github_link_id.
2. Remove all @Suppress("DEPRECATION") annotations from the 7 files above.
3. Remove the @Deprecated annotations and the deprecated fields/parameters (githubLinkId in Workspace, CreateWorkspaceCommand, WorkspaceDtos, WorkspaceController).
4. Verify no test breaks (tests already suppress the deprecation and may need updating).

Files involved:

• New V22__drop_github_link_id.sql
• domain/model/Workspace.kt, application/command/CreateWorkspaceCommand.kt, CreateWorkspaceCommandHandler.kt
• infrastructure/web/WorkspaceController.kt, infrastructure/web/dto/WorkspaceDtos.kt
• infrastructure/persistence/JooqWorkspaceRepository.kt
• test/…/CreateWorkspaceCommandHandlerTest.kt, test/…/WorkspaceControllerTest.kt