docs(decisions): PDR-004 resolved, KD-001 closed, KD-002 re-scoped (S5-006)

[lwalden]

Summary

Sprint S5-006 — DECISIONS.md hygiene. Metadata/decision hygiene only — no code changes.

• PDR-004 marked RESOLVED: per-sleeve paper-validation gate landed in S4-001 (PR feat(config): per-sleeve paper-validation thresholds (S4-001) #81), owner-confirmed 2026-06-09; resolution text records the thresholds of record and the IConfigRepository settings seam (key "sleeveValidationThresholds").
• KD-001 closed: Discord webhook wired by S5-004 day-0 (test alert delivered HTTP 204 through the production guard path).
• KD-002 re-scoped: Claude API key not required under default gateway-or-rules posture (DirectApiFallbackEnabled=false); relevant only if the owner enables the metered fallback.

Test plan

• Docs-only diff (DECISIONS.md); CI build skipped by docs-only path filter.
• Verified the settings-seam pointer matches SleeveValidationThresholds.SettingsKey in src/TradingSystem.Core/Configuration/SleeveValidationThresholds.cs.

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