feat: normalize toxicity and add pet-safe filter

[hualuoz]

Summary

• normalize legacy missing, string, and list toxicity values into a canonical tag list
• add plantguide species list --pet-safe with conservative filtering and a veterinary disclaimer
• test normalization precedence, the full catalog schema, CLI filtering, and disclaimer output
• replace the invalid toxicity CLI draft with user-facing schema and safety documentation

Bounty

• Type: feature
• Expected reward: 50 MRG
• Bounty: [50 MRG] Toxicity and pet-safety flag pack across catalog #20
• Claim: [50 MRG] Toxicity and pet-safety flag pack across catalog #20 (comment)
• Claim intake: Claim MRG Tokens for Bug Bounty Reports - Comment New Bugs Here Before Opening a PR mergeos#1 (comment)

Why

The catalog currently mixes three toxicity shapes, so callers cannot reliably filter it. Some records also contain both safe and toxic wording. Normalization now guarantees exactly one pet-safety state (pet_safe, toxic_to_pets, or unknown), with toxicity warnings taking precedence and ambiguous records remaining unknown.

User impact

Users can list only explicitly pet-safe catalog entries while uncertain entries are excluded. The output makes clear that catalog flags are informational and should be verified with a veterinarian or trusted toxicology source.

Validation

• pytest -q — 59 passed
• ruff check — passed
• ruff format --check — passed
• plantguide species list --pet-safe — verified against the bundled catalog

No secrets, production keys, or private customer data are included.

Fixes #20

[hualuoz]

@TUPM96 This implements the 50 MRG bounty in #20 and is ready for review. The full suite passes (59 tests), and ambiguous toxicity records are excluded from the pet-safe filter.