feat: add --domain option to doctor, backfill versioning files

[iracic82]

Summary

• Add explicit --domain parameter to doctor across all three interfaces (CLI, Python API, MCP)
• Remove hardcoded default domain — discovery check is skipped with helpful hint if no domain specified
• Backfill CHANGELOG entries for 0.6.9, 0.7.0, 0.7.1, 0.7.2 and fix all missing compare links
• Update CITATION.cff (was stuck at 0.6.8) and CONTRIBUTING.md (test count, NIOS backend, version file list)
• Bump version to 0.7.3

Usage

# CLI
dns-aid doctor --domain example.com

# Python
from dns_aid.doctor import run_checks
report = run_checks(domain="example.com")

# MCP
diagnose_environment(domain="example.com")

# Env var fallback
export DNS_AID_DOCTOR_DOMAIN=example.com
dns-aid doctor

Test plan

• 750 tests pass
• ruff check clean
• dns-aid doctor without domain shows skip hint
• dns-aid doctor --domain example.com resolves against specified domain
• dns-aid doctor --help shows --domain option