fix(docs): rdd-shadow-evaluation.md references retired GENTLE_AI_RDD_SHADOW switch #2998

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📝 Bug Description

docs/architecture/rdd-shadow-evaluation.md still documents the GENTLE_AI_RDD_SHADOW=1 opt-in switch as current, operator-facing guidance, but the switch and its shadow observer were removed from the codebase in RDD root simplification Wave 7 S2a (commit 17e40eb0, "retire the Wave 1 shadow observer surface").

On current upstream main (69ac5a45), GENTLE_AI_RDD_SHADOW appears in the code only in comments stating it was retired, for example internal/reviewtransaction/candidate_relation.go:8: "the Wave 1 shadow observer that also called it (shadow_observer.go, GENTLE_AI_RDD_SHADOW) retired in Wave 7 S2a". There is no environment-variable reader left, and shadow_observer.go no longer exists. The doc remains as an actionable "Quick path" that tells readers to set the switch and watch stderr for a gentle-ai.rdd-shadow/v1 line that can never be emitted.

🔄 Steps to Reproduce

1. Check out current upstream main (69ac5a45).
2. Read docs/architecture/rdd-shadow-evaluation.md, "Quick path" section (lines 7-8): "To opt in for local investigation, set GENTLE_AI_RDD_SHADOW=1 before running a gate".
3. Grep the codebase for GENTLE_AI_RDD_SHADOW: only comments remain, all saying the switch was retired in Wave 7 S2a.
4. Run any gate with GENTLE_AI_RDD_SHADOW=1 set: no gentle-ai.rdd-shadow/v1 line is ever written, because shadow_observer.go and shadowObservationEnvVar were deleted in Wave 7 S2a (commit 17e40eb0).

✅ Expected Behavior

docs/architecture/rdd-shadow-evaluation.md is updated or removed so it no longer instructs users to set a switch that does not exist. If shadow evaluation is retired, the doc should say so (or be deleted), matching the Wave 7 outcome.

❌ Actual Behavior

The doc presents a removed switch as current guidance. A reader following it sets an environment variable that has no effect and waits for stderr output that cannot appear.

🖥️ Environment

• Gentle AI Version: main @ 69ac5a45 (documentation-only defect; no installed release involved)
• Operating System: Linux (Arch/Manjaro)
• AI Agent / Client: Other
• Affected Area: Documentation

💡 Logs / Error Output

N/A - documentation-only report; no runtime logs.

Additional Context

• Evidence of removal: commit 17e40eb0 ("feat(reviewtransaction): retire the Wave 1 shadow observer surface", Wave 7 WU4+WU5+WU6) removes ObserveShadowRelation (shadow_observer.go), shadowObservationEnvVar, and all 5 consumers. It is an ancestor of upstream main.
• The Wave 7 change (openspec/changes/rdd-root-simplification-wave7) records the observer as fully retired and does not plan any update to docs/architecture/rdd-shadow-evaluation.md, so this doc is stale rather than intentionally historical.
• This is a post-removal documentation accuracy defect (the removal has landed on main); it is not reporting an in-flight or planned redesign mismatch.