feat(vouching): fix #4 by adding mentor vouching contract

[Dopezapha]

Closes #4

PR Description

This issues closes issue #4 by adding a standalone mentor vouching contract for StepFi learners. Verified mentors can now vouch for a learner, which stores an on-chain vouch record and calls the reputation contract to add a score boost. Mentors can also revoke an active vouch, which marks the record inactive and removes the same reputation boost.

The flow now starts with admin mentor verification through set_mentor. A verified mentor calls vouch, the contract rejects duplicates, writes a timestamped VouchRecord, indexes it by learner for lookup, emits MENTORVOUCHED, and calls add_boost on reputation. Revocation requires the same mentor auth, rejects missing or inactive records, calls remove_boost, persists active = false, and emits VOUCHREVOKED.

This matters because first-time learners now have a social trust path before loan history exists, while mentor authorization and reputation updates remain on-chain.

Changes Made

• Added contracts/vouching-contract for issue [4] Add mentor vouching contract #4 with lib.rs, types.rs, storage.rs, events.rs, errors.rs, tests.rs, and Cargo.toml.
• Added verified mentor storage and mentor/learner vouch records for issue [4] Add mentor vouching contract #4.
• Added learner-indexed vouch lookup through get_vouches.
• Added exact issue [4] Add mentor vouching contract #4 event topics: MENTORVOUCHED, VOUCHREVOKED, and MENTORVERIFIED.
• Added duplicate-vouch, unverified-mentor, revoke, admin, event, and mock reputation cross-contract tests.
• Added add_boost and remove_boost to contracts/reputation-contract/src/lib.rs.
• Added reputation boost tests in contracts/reputation-contract/src/tests.rs.
• Added vouching-contract to the workspace Cargo.toml.
• Updated context/progress-tracker.md for issue [4] Add mentor vouching contract #4.