fix(workbuddy): sanitize Claude Code billing attribution headers - #482

Summary

• Add x-anthropic-billing-header and x-anthropic-billing-hdr to the existing WorkBuddy WAF request-body sanitizer (sanitizeFingerprintText / sanitizeWAFKeywords in payload.go)
• Both spellings are stripped as a whole line (including trailing cch= params), with the embedded and bare-key fallbacks from hub #855 / v2.0.8
• A system/developer message that is only that header is dropped via the existing empty-message cleanup, and ensureLeadingSystem restores a non-empty leading system prompt if needed (avoids 400 / 11151)
• Regression tests cover both keys, case-insensitivity, other fields left untouched, header-only system drop, and the PrepareBody outbound path

Aligns with workbuddy2api-hub #855 (landed in v2.0.8, tip 7ffba72). Claude Code 2.1.37+ injects these reserved keys into the first system text block; sending them as-is is rejected with 400 / 11128.

Out of scope

• orphan call_id, tool_search, gpt-image-2.6, SIWC, cloak, Muse
• Qoder tool stream
• CPA 714f052 image_generation injection pipeline
• domestic desktop or conversation CLIVersion
• panel / usage cache

Test plan

• go test ./internal/providers/workbuddy/ -count=1
• go vet ./internal/providers/workbuddy/
• python3 scripts/release-notes.py validate
• Confirm merge/release wait for separate user nod; do not deploy SGP3