[rush reporter][R9] Complete Rush 6 reporter and plugin migration

Parent: #5858

Blocked by: #5977, #5979, #5980, #5981; coordinated with #5900

Goal

Execute the daemon-aligned Rush 6 reporter cutover after the opt-in path and compatibility gates are proven.

Deprecate and remove ILogger.terminal and RushSession.terminalProvider in the major release.
Gate incompatible plugins before apply() with the structured migration diagnostic.
Retain the legacy renderer, AlreadyReportedError bridge, verbosity aliases, and RUSH_REPORTER=legacy emergency fallback for at least one major.
Enable default reporter selection and environment-based agent selection in coordination with the rushd client/default flip.
Validate daemon and in-process clients consume the same presentation-free event stream and derive the same exit behavior.

Plugin incompatibility is detected before plugin code executes.
Rush 5 compatibility behavior and Rush 6 breaking behavior are explicitly tested.
Reporter and rushd cutovers ship in the same major and pass [rushd][WS5] Rush 6 cutover: testing, observability & migration #5900 transparency gates.
The legacy emergency path remains documented, tested, and independent of the daemon fallback.
Default flip is reversible to the opt-in behavior without protocol rollback.

Unblocks R11.

Part of #5858.