[rush reporter][R2] Add opt-in controls and the frontend ReporterHost

Parent: #5858

Blocked by: #5974

Goal

Create the authoritative frontend reporter host and reversible pre-major activation controls, while preserving legacy output by default.

Construct ReporterHost/ReporterManager in apps/rush before Rush version selection.
Register --reporter, --output, --log-level, --quiet, --verbose, and --debug through the supported command-line surface.
Read reporter configuration and agentEnvironmentVariables from rush.json.
Add an experiments.json opt-in (proposed name: useRushReporter).
Treat either an explicit non-legacy --reporter or the repository experiment as a deliberate pre-major opt-in; do not enable agent auto-selection yet.
Preserve RUSH_REPORTER=legacy as the emergency override.

The frontend owns reporter selection and hands the engine only a typed sink.
No opt-in produces the current legacy output and behavior.
Explicit and repository-level opt-ins are deterministic and covered across TTY, non-TTY, and CI inputs.
Unsupported or contradictory controls fail with an actionable diagnostic.
Agent detection remains disabled as a default-selection mechanism until R8.

Unblocks R5 and R6.

Part of #5858.