feat(advisor): add GraphMentorMatchAdvisor - agentic mentor/mentee pairing

9th agentic sibling. For each low-degree / isolated mentee, recommends up to N mentor candidates from the graph using a deterministic per-pair score (shared-neighbour bridge, mentor load, clustering coefficient, degree median, direct-connection / mentee penalties, risk-appetite scaled). Greedy assignment respects per-mentor cap; mentors flagged PRIME_MENTOR / WEAK_MENTOR / OVERLOADED / NOT_SUITABLE. Plan exposes matched/weak/unmatched fractions, mentor utilization, bridge-match fraction, playbook (RECRUIT_EXTERNAL_MENTORS, PAIR_UP_ISOLATED_MENTEES, LAUNCH_MENTORSHIP_PROGRAM, REDISTRIBUTE_OVERLOADED_MENTORS, WARM_INTRODUCE_VIA_SHARED_FRIEND, EXPAND_MENTOR_TRAINING, SOLICIT_NEW_MENTEES, SCHEDULE_FOLLOWUP_AUDIT, HEALTHY_MENTORSHIP_PIPELINE), insights, and A-F grade. Pure JDK + JUNG, single file, deterministic with Clock, JSON renderer with stable key order, never mutates input. Includes 13-case JUnit suite covering empty graph, no-mentors-available, isolated-match, mentor cap / OVERLOADED, bridge boost, direct-connection penalty, isolated playbook action, CAUTIOUS/AGGRESSIVE risk-appetite behaviour, grade boundaries, healthy fallback, JSON determinism + key order, and graph-immutability.

Author: sauravbhattacharya001