Tracked current-registry built-in provenance through AgentSession so plan mode
only force-activates the built-in write implementation. Extension or SDK tools
that shadow the name `write` stay inactive, preserving plan mode's read-only
contract through the built-in write/edit guard.
Added a regression that registers a shadowing write tool without built-in
provenance and verifies plan mode does not activate it.
roboomp
6f3d6ba2e4