Enable agent host by default for insiders (#318007)

* Enable agent host by default for insiders

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Ignore local agent host setting in web

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix agent host browser contribution gates

Keep shared browser-layer contributions gated on the raw setting so desktop/browser tests continue to exercise their behavior, while the web runtime ignore remains in the service/client paths.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>