[rush reporter][R8] Qualify AI reporter auto-selection

[TheLarkInn]

Parent: #5858

Blocked by: #5977, #5978

Goal

Prove that the AI reporter improves agent outcomes before enabling environment-based automatic selection.

Scope

• Build a deterministic corpus of representative install, configuration, dependency, build, cache, network/auth, and plugin failures.
• Compare legacy/plaintext/AI output for remediation success, context consumption, determinism, secret handling, and full-log discoverability.
• Validate built-in COPILOT_CLI detection plus configured agentEnvironmentVariables.
• Define pass/fail thresholds and store machine-readable gate results.
• Keep auto-selection disabled until the corpus gates pass.

Acceptance criteria

• The corpus is deterministic and runnable in CI.
• AI output never includes classified secrets and does not regress producer privacy.
• Thresholds for remediation quality, output size, determinism, and log-path correctness are documented and enforced.
• Passing the gate enables agent auto-selection only on the opt-in reporter path before Rush 6.
• RUSH_REPORTER=legacy remains authoritative.

DAG

Unblocks R9.

Part of #5858.

[TheLarkInn]

R8 qualification integration is reviewed and fixed in #5999 at ac3fd75, but this issue remains blocked.

The corpus now measures actual reporter output and AI log discoverability, validates exact context values/privacy markers, uses scenario-specific deterministic evidence with aggregate/per-case/compact size gates, preserves honest secret-diagnostic counts, and explicitly requires the separate privacy prerequisite before future AI eligibility. Three standalone runs were byte-identical; all blocking corpus gates passed locally, and reporter/rush-lib/apps-rush validation passed.

Remaining gates: #5990 must merge and be accepted; stacked base #5997 must merge; #5999 must receive required CI/review and merge with auto-merge still disabled while stacked; the qualification must pass on main; only then may approved opt-in activation consume both prerequisites. RUSH_REPORTER=legacy remains authoritative and no Rush 6/default activation was introduced.