Add Rush reporter repository configuration

[Sean Larkin (TheLarkInn)]

Part of #5975.

Stack

• parent: [rush reporter][R1B] Generate install-run-rush bootstrap protocol #5986 (copilot/reporter-r1b-bootstrap-generation)
• grandparent: Wire rush-reporter package dependencies #5985 (copilot/reporter-r1a-package-wiring)

This is the R2A child of #5986. Keep auto-merge disabled while either ancestor is open. After #5985 and #5986 merge in order, retarget this PR to main, verify its slice and CI, then merge it before #5989.

Changes

• Add the experimental useRushReporter opt-in to experiments types, schema, and rush init template.
• Add typed rush.json loading for reporting.agentEnvironmentVariables, including schema and template support.
• Add focused default, parsing, and invalid-input coverage plus the Rush API report and changefile.

Validation

• rush test --only @microsoft/rush-lib
• rush check
• rush change --verify --no-fetch
• Generated rush init files verified to omit reporting and useRushReporter by default.
• gh pr diff 5987 contains only the 11-file R2A configuration/schema slice.

Compatibility and non-goals

Absent or false useRushReporter preserves legacy reporting behavior, and absent reporting configuration yields an empty agent-variable list. This PR only exposes typed repository configuration; it does not activate agent auto-selection, add frontend/CLI controls, or produce visible reporter output.

[Sean Larkin (TheLarkInn)]

Independent stack-base review found no actionable defects in the repository configuration/schema/API slice. Local rush-lib tests, rush check, and changefile verification passed. Combined validation with #5989 is clean. Stack gate: keep this PR based on copilot/reporter-r1a-package-wiring with auto-merge disabled until #5985 merges; then retarget to main and run full CI. Merge #5987 before #5989 so the public useRushReporter schema/API lands before the frontend consumes it.

[Sean Larkin (TheLarkInn)]

Linear stack update: R2A is now head c647c7f based on #5986. Its GitHub diff is exactly the 11-file configuration/schema slice. Auto-merge remains disabled while #5985/#5986 are open.

[Sean Larkin (TheLarkInn)]

Combined deep review of current head c647c7f967.

No critical, high, or medium actionable findings. No useful low follow-ons.

[Sean Larkin (TheLarkInn)]

Deep review of current head e191fa7 found no critical, high, or medium actionable issues. The only failed matrix job was a wall-time benchmark flake, and its failed job has been rerun. Merge remains gated by maintainer approval and rerun success.

[Sean Larkin (TheLarkInn)]

The isolated wall-time benchmark rerun passed. Current head e191fa7 is full-CI green and blocked only by maintainer approval.