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

[TheLarkInn]

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.

Scope

• 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.

Acceptance criteria

• 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.

DAG

Unblocks R11.

Part of #5858.