GitHub integration: board-level sync rules and per-board repository routing

Bring the GitHub integration in line with Linear and Jira: let each Featurebase board (category) route to its own GitHub repository, and let sync rules be scoped per board.

How it works today

Automatic push uses one repository for the whole workspace. It can be limited to selected categories, but every pushed post lands in that single repository.
GitHub sync rules map a GitHub issue state to a Featurebase status. A rule has no board field and no repository field, so rules cannot be scoped by board or by repo.
The repository is only selectable at push time, on a manual push. A post can end up linked to issues in several repositories that way, one push at a time.
One GitHub App installation per workspace.

Requested behaviour

A repository field and a board field on GitHub sync rules, so each board can drive its own repository and its own status mapping.
Automatic push that resolves the target repository from the post's board instead of from one global setting.
Same shape as Configure Linear integration Sync Rules per board and Jira integration sync rules with multiple projects, both already shipped.

Why it matters

Raised by a prospect running roughly 15 products, each with its own engineering team and its own repository. Without board-to-repository routing, only one team gets hands-off automatic sync and every other team has to push manually, or the company has to split into separate Featurebase workspaces purely to work around the integration.