fix(openclaw-session-bloat-warning): gate early warnings by observed usage

[TeodorArg]

Summary

• gate char and message heuristics so observed provider usage below token pressure does not trigger early visible warnings
• treat inconsistent cached and total usage as diagnostic-only context instead of warning evidence
• align copy, config metadata, fallback context-window defaults, and tests with the new warning behavior

Verification

• cd openclaw-session-bloat-warning && pnpm check