feat: add disease and pest symptom matcher

[hualuoz]

Summary

• replace the species-ranking placeholder with an offline symptom-to-issue matcher
• add a five-issue JSON fixture covering scale, aphids, spider mites, fungal leaf spot, and root rot
• return likely causes, practical remedies, escalation guidance, and an educational disclaimer
• keep the existing plantguide identify disease entry point and improve its help text

Verification

• python -m pytest -q → 51 passed
• python -m pytest tests/test_disease.py -q → 4 passed
• ruff check src/plantguide/identify/disease.py src/plantguide/cli.py tests/test_disease.py → passed
• ruff format --check src/plantguide/identify/disease.py tests/test_disease.py → passed
• git diff --check → passed
• plantguide identify disease --symptoms "yellow spots,sticky leaves" --top 3 → ranks scale_insects first at 1.0, with both symptoms, causes, remedies, escalation guidance, and the disclaimer

Fixes #6