Add opinionated environment variables guide to AFL++ skill

[dguido]

The skill documents compile-time variables (AFL_USE_ASAN, AFL_LLVM_CMPLOG) but lacks practical guidance on runtime environment variables. Rather than duplicating the official docs, add an opinionated "how to use them right" section based on real-world experience.

Suggested Section: "Environment Variables That Matter"

Always Set These

# Every campaign should use tmpfs - SSDs will thank you, and it's faster
AFL_TMPDIR=/dev/shm

# 2.5x faster calibration with negligible precision loss
AFL_FAST_CAL=1

Why: These are free performance wins with no downsides. Not setting AFL_TMPDIR wears out your SSD and slows fuzzing. Not setting AFL_FAST_CAL wastes time on startup for marginal precision gains.

Multi-Core Campaigns

# On the primary (-M) instance only
AFL_FINAL_SYNC=1

# On all instances - share findings faster
AFL_TESTCACHE_SIZE=100

Why: Without AFL_FINAL_SYNC, your primary instance might miss late-discovered paths from secondary instances. Default cache is too small for large campaigns.

CI/Automated Fuzzing

# Fail fast if fuzzing isn't finding anything
AFL_EXIT_ON_TIME=3600  # 1 hour with no new paths = stop

# Or run until "done" (all queue entries processed)
AFL_EXIT_WHEN_DONE=1

# Headless environments
AFL_NO_UI=1

Why: Unbounded fuzzing in CI wastes resources. Set time limits or use exit conditions.

Variables to Avoid

Variable	Why Skip It
AFL_NO_ARITH	Rarely helps, can hurt coverage
AFL_SHUFFLE_QUEUE	Only for exotic setups, usually harmful
AFL_DISABLE_TRIM	Trimming is valuable, don't disable without reason

Reference

Full list: https://aflplus.plus/docs/env_variables/ (but most are niche - stick to the ones above)

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Follow-up from PR #15 review feedback.