Add did:trail method checklist submission

[chrishooooo-netizen]

Completed DID Method Checklist self-assessment for did:trail, a DID method for AI agent and organizational identity with an HTTP-based trust registry (no blockchain) plus an offline self-signed mode (did:trail:self).

• Added: submissions/did-trail.md
• All 18 sections (0-17) completed; every R1-R22 answer carries a Yes/No/Partial verdict with a spec section reference.
• Two requirements are answered "No (by design)" rather than polished away: R11 (Cannot Be Administratively Denied) and R20 (Registry Agnostic). The method uses an authoritative registry on purpose for accountability, with a registry-free self mode as the counterweight. Both are documented as deliberate trade-offs.
• Spec reference: spec/did-method-trail-v1.md v1.2.1 (https://github.com/trailprotocol/trail-did-method).

Note: the README references scripts/validate-submission.mjs, schema/, and examples/, which are not present in the current repo, so this submission was self-validated against checklist.md (section-for-section parity confirmed). Happy to adjust the file path or format to match whatever submission layout you settle on.