[Feature] Let feedback indexing scope discovery by pull-request state

[morluto]

Problem

github.index_pull_request_feedback currently enumerates pull requests with state=all. That deliberately preserves historical feedback, but it prevents an agent from requesting an index limited to the current open review surface.

The offline feedback search supports a state filter, but it runs only after acquisition. It cannot avoid listing, fetching, and persisting closed or merged pull requests.

This matters for workflows such as “review all open PRs and summarize feedback from a particular reviewer”: the agent can identify an exact open set, but the repository-wide index still spends its request and persistence budget on historical PRs. The exact-sync tool is an alternative for a known set, but it is capped at 50 inputs (#284), so it is not a general state-scoped index.

Requested outcome

Allow callers of github.index_pull_request_feedback to choose the pull-request state to discover, for example open, closed, or all. Keep all as the default so existing historical-coverage workflows retain their current behavior.

Success criteria

• An open request asks the provider only for open pull requests and reports that selected scope in job progress/result metadata.
• Index/checkpoint/coverage semantics retain the selected state, so an open-only run is never presented as complete historical coverage.
• The existing default remains all and remains backward compatible.
• Offline search filtering continues to work, but acquisition is no longer needlessly broader than the caller’s requested scope.
• Tests cover the provider request and scoped coverage/checkpoint behavior.

Notes

This is a workflow-control improvement, not a request to discard historical feedback. Historical indexing remains useful; agents should be able to choose it rather than having it imposed for every repository-wide feedback operation.