[gateway] Map GitHub PR labels to Wave complexity points

[yinkscss]

Context

parse_merge_event hardcodes points: 1 for every merged PR. PRD open question OQ1 defaults to fixed points, but Drips Wave uses complexity tiers (100/150/200 base points).

Problem

Wave-style programs cannot differentiate trivial vs high complexity rewards without manual ratio changes per contributor.

Proposed scope

• Parse PR labels from webhook payload (e.g. complexity:trivial, complexity:medium, complexity:high)
• Map labels to configurable point values via env or shared config
• Fall back to 1 point when no complexity label is present (preserve v1 default)
• Document label convention for maintainers adding issues to Wave programs

Acceptance criteria

• Merged PR with complexity label assigns mapped points to attestation
• Unlabeled merges still use default 1 point
• Invalid or conflicting labels fail closed with audit log entry
• Mapping documented for Stellar Wave maintainers

References

• crates/gateway/src/webhook.rs (parse_merge_event, points: 1)
• crates/shared/src/types.rs
• docs/PRD.md OQ1
• Drips Wave maintainer docs (complexity tiers)