fix: gracefully handle missing workspace.branch column after OpenCode v2 migration

[bet4it]

Running OpenCode v2 applies a workspace_domain migration that replaces the workspace table columns with provider bindings, removing the branch column. The old code only checked for the table's existence, so after a v2 run the query SELECT w.branch fails with "no such column". Switch to pragma_table_info('workspace') to check for the branch column itself, matching the existing pattern used for the version column in the same file. When the column is absent the branch info is simply omitted (git_branch = None), which is consistent with how Codex, Cursor, Kimi, Pi, and other providers behave.