fix: hide write UI resources in read-only mode

[he-yufeng]

Summary

• pass the server read-only setting into MCP App UI resource registration
• keep the read-only get_me_ui resource available
• skip issue_write_ui and pr_write_ui when write tools are filtered by --read-only

Fixes #2519.

To verify

• go test ./pkg/github -run "TestRegisterUIResources_ReadOnlySkipsWriteResources|TestRegisterUIResources_ReadableViaClient|TestNewMCPServer_RegistersUIResources" -count=1
• go test ./pkg/github -count=1
• git diff --check

[he-yufeng]

Validated the current branch locally after closing the superseded #2574:

go test ./pkg/github -run UIResources
git diff --check origin/main...HEAD

[he-yufeng]

Rebased on current main in b8743ed.

Validation:

• go test ./pkg/github -run UIResources
• git diff --check origin/main...HEAD