Closes #17: Implemented: add maintainer review checklist for Stellar and Soroban issues #17

[Promise278]

Closes #17

Summary

Rewrites the maintainer review checklist into a two-phase structure: Phase 1 (code review required for merge) and Phase 2 (GrantFox reward-readiness, completed post-merge by campaign reviewer).

What changed

• docs/MAINTAINER_REVIEW_CHECKLIST.md — comprehensive rewrite with 8 code review categories + GrantFox reward-readiness section
• .github/PULL_REQUEST_TEMPLATE.md — updated to reference Phase 1 only

New check categories (Phase 1)

• Stellar network correctness — branded types, amounts, memos, assets, balance model, readiness, Horizon errors, mainnet gate
• Anchor and SEP flow correctness — metadata schemas, callbacks, lifecycle state machine, status exhaustiveness
• Payment intent and readiness correctness — schema validation, readiness stages, balance integration
• Soroban contract correctness — admin auth, status DAG, evidence write-once, dispute/release guards, events, summary math
• Secret leakage (R0–R6) — all 7 rules + diff search
• Mainnet safety — testnet defaults, assertNetworkAllowed
• Test coverage — positive/negative cases, branch coverage, shared fixtures
• Documentation — topic docs, README index, security docs

GrantFox separation

Phase 2 is clearly marked as post-merge, completed by the campaign reviewer. Covers scope match, code/test quality, security posture, documentation completeness, and reward-readiness verdict.