Fix IFC private repository labels

[RossTarrant]

Summary

• classify private repository issue/list/search results as private-trusted instead of private-untrusted
• update search-related IFC comments and expectations for joined private results
• add focused IFC unit coverage for list/search issue labels

Tests

• script/lint && script/test
• go test -race ./pkg/ifc ./pkg/github

[Copilot]

Pull request overview

This PR updates the IFC (Information Flow Control) labeling behavior for issue list/search (and other multi-repo search-style outputs) so that private repository results are labeled private-trusted rather than private-untrusted, and adjusts related documentation/comments plus unit tests accordingly.

Changes:

• Update pkg/ifc label helpers to return PrivateTrusted() for private issue list/search results.
• Refresh IFC-related comments in search/list code paths to reflect the new private-trusted semantics.
• Add/adjust unit tests to assert the updated IFC integrity/confidentiality expectations.

Show a summary per file

File	Description
pkg/ifc/ifc.go	Changes private issue list/search label functions to return PrivateTrusted() and updates associated IFC join documentation.
pkg/ifc/ifc_test.go	Adds focused unit coverage for list/search label integrity/confidentiality outcomes.
pkg/github/search.go	Updates inline comments describing IFC joining behavior for repository/code/commit searches.
pkg/github/search_test.go	Updates IFC insiders-mode expectations for search_repositories when private repos are present.
pkg/github/repositories.go	Updates inline comments for joined IFC labeling in starred-repo listing.
pkg/github/issues_test.go	Updates IFC insiders-mode expectations for issue read/search/list flows involving private repositories.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 7

[JoannaaKL]

Can we move attachProjectVisibilityIFCLabel to ifc.go? We can fetch isPrivate and pass it to the function