docs: WHY_ALMANAC — differentiation vs data.gov and other registries

[rudi193-cmd]

Problem

Adversarial review: unclear value prop vs data.gov, AWS Open Data Registry, and CKAN portals. "Monitoring" alone is not differentiated if nobody acts on alerts.

Acceptance

• docs/WHY_ALMANAC.md in template (verticals link from README)
• Covers: catalog-don't-host, reachability monitoring, YAML+schema portability, vertical curation vs portal search
• Honest limits: we don't replace portals; we maintain a small verified map with operational status

Related

• climate-almanac governance: CODEOWNERS.example + steward rollout checklist #5 (status page epic), feat: revised-vs-superseded disambiguation via lead-signature fingerprint (almanac-template#11 item 3) #20–#21 (monitoring tooling)
• Complements feat: archive-rot recheck for recovery[] candidates (almanac-template#11 item 2) #19 (GOVERNANCE.md)

Context

Adversarial stack review (Nest/adversarial.txt, 2026-06-30) §3