Add per-client CHANGELOG.md, maintained by googly.release

[mhsdef]

What

Adds a per-client CHANGELOG.md to each generated client, surfaced where Hex/Elixir users actually look, and maintained automatically by mix googly.release.

• docs: [extras: […, "CHANGELOG.md"]] → hexdocs shows a CHANGELOG tab at the point of install.
• package: [files: […, "CHANGELOG.md"]] → the changelog ships inside the published tarball, reproducible with the version.
• Googly.Changelog (new module) is the single source of truth for the changelog format.

How it fits the generator's invariants

The changelog is accumulated history, so it's treated like each client's @version: it lives in the committed client dir and is preserved across regeneration — mix googly.generate never clobbers it, it only lays down a baseline for a brand-new client (Changelog.ensure/3). mix googly.release prepends a dated entry on each bump (Changelog.add_release/4). It carries no "auto generated" banner, precisely because a human may refine an entry before publishing.

Backfill

All three existing clients get:

• ## 0.1.0 — initial release
• ## 0.1.1 — RFC 6570 {+name} fix, deterministic optional-param ordering, "Google"-prefixed description

Changed files

• lib/googly/changelog.ex — new module (baseline / prepend / format)
• lib/googly/generator.ex — write_package_files ensures a baseline changelog
• lib/googly/generator/renderer.ex — version/1 made public for reuse
• lib/mix/tasks/googly.release.ex — bump prepends a dated changelog entry
• templates/client/mix.exs.eex — CHANGELOG in extras + files
• test/googly/generator_test.exs — asserts CHANGELOG.md is scaffolded
• clients/*/{CHANGELOG.md,mix.exs,README.md} — regenerated output + backfill

Not included (by design)

• GitHub Releases — for an N-independently-versioned monorepo they'd need per-package tags in one namespace and duplicate what hexdocs already shows.
• Discovery-surface diffing — specifications/gdd/*.json is gitignored, so there's no committed prior surface to diff against. Release entries use a generic default note you edit before committing. Adding a per-client surface manifest to diff against is a possible follow-up.

Testing

• mix test → 60 passed
• mix compile --force --warnings-as-errors (CloudStorage) clean
• mix format clean
• Regeneration diff is isolated (mix.exs extras/files + README ~> 0.1.0→0.1.1)
• Ran a real mix googly.release CloudStorage: bumped 0.1.1 → 0.1.2 and prepended ## 0.1.2 above preserved history, then restored the files