[Docs] No cross-contract error-code reference; numeric contracterror codes collide across the four contracts

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

Each contract numbers its contracterror enum independently from 1: e.g. code 7 = InvalidRepaymentAmount in RemittanceNFT (lib.rs:16), InvalidAmount in LoanManager (lib.rs:42), and InsufficientLiquidity in LendingPool (lib.rs:19); only GovernanceError namespaces (4001+). The backend/indexer decode raw numeric codes from any of the four contracts, so a decoded code N is ambiguous and there is no single doc mapping code -> meaning per contract (lending_pool also skips code 8). No error-code reference file exists in the repo.

Acceptance criteria

• Add an error-code reference (e.g. contracts/ERROR_CODES.md) tabulating every contracterror variant, its numeric code, and contract of origin
• Recommend/adopt per-contract numeric namespacing (as multisig already does) or document why codes are reused
• Reference the doc from contracts/README.md

Files to touch

• contracts/remittance_nft/src/lib.rs
• contracts/loan_manager/src/lib.rs
• contracts/lending_pool/src/lib.rs
• contracts/multisig_governance/src/lib.rs

Out of scope

• Replacing panics with enums (already done)
• Backend API error codes