chore(release): prepare 0.0.20

[DavidBabinec]

What

Prepares the 0.0.20 release: the dated changelog section, the version bump, and the three deployment docs that intentionally pin a semver image tag.

The changelog section is built from the 19 PRs merged into main since v0.0.19. Every commit in v0.0.19..main is a PR merge, so the roster and the commit range reconcile exactly. Three PRs earn no entry: #528 (branch entity adapters, an internal refactor with no user-visible change), plus #515 and #516 (Windows-only test-gate fixes).

Headline changes in the release: site branches with a merge review page and page version history (#484), the Dependencies panel becoming an npm registry browser (#489), and a publish gate that names the file blocking the publish (#533). Eleven bug fixes, including two that lost data: the collab relay wiping SEO and plugin cells on edit (#504), and a plugin upgrade breaking every published page's frontend assets (#359).

No security advisories ship with this release, so there is no Security section.

Why

Release preparation is a separate PR so the tag points at a commit that describes the release it is. Nothing else belongs here.

Impact

• Users: none from this PR. Upgrading to 0.0.20 runs migrations 027 through 030, all additive (418 insertions, no deletions across both dialects).
• Operators: the deployment docs now pull ghcr.io/corebunch/instatic:0.0.20. The Render blueprints stay on :latest, and scripts/build-release-bundle.ts rewrites the bundle copies to the semver tag.

Verification

bun run build   # exit 0
bun test        # exit 0, 7053 pass, 0 fail
bun run lint    # exit 0