Bring AtmosphericModels' changelog to the release convention #54

Bring AtmosphericModels' changelog to the release convention

Bring CHANGELOG.md to the one convention the box releases from, so agent release / "release " on Slack opens the bump PR without a refusal (1-Bart-1/Agents#608). Pending notes sit under one ## Unreleased header above every release (or as files in changelog.d/), grouped under ### Added / ### Fixed / ### Changed. Every released section is headed ## <PackageName> vX.Y.Z YYYY-MM-DD: ## level, v before the version, ISO date, no dash, no brackets, no "(unreleased)". Rewrite headers only; leave the note text as it is. Check: bin/release --dry-run (the box copy, rules/release in 1-Bart-1/Agents) on the PR head reads the version and the notes.

In this repo: pending notes sit under ## AtmosphericModels v0.3.13 (unreleased), which the bump does not read as unreleased; releases use vX.Y.Z - YYYY-MM-DD.