Add ASM manifest for Cohere

[YE-YI7]

Add a source-linked ASM manifest for Cohere.

Use this existing manifest as the template:
https://github.com/calebguo007/asm-spec/blob/main/manifests/openai-gpt-4o.asm.json

Acceptance criteria:

• Add one new manifests/cohere-*.asm.json file.
• Include asm_version, service_id, taxonomy, and display_name.
• Fill pricing.billing_dimensions from a public Cohere pricing/docs URL.
• Fill provenance.source_url, retrieved_at, last_verified_at, and verification_status.
• Add quality metadata only if there is a public benchmark/source; do not invent benchmark claims.
• Run python -m pytest scorer/test_manifests_schema.py -q and include the result in the PR.

Useful docs:

• Manifest template: https://github.com/calebguo007/asm-spec#manifest-template
• Contributing guide: https://github.com/calebguo007/asm-spec/blob/main/CONTRIBUTING.md