feat(release-manifest): add the release.json schema

[zgeoff]

Contributes to GEO-1058

Description

Adds @zgeoff/release-manifest, a draft 2020-12 JSON Schema for the release.json that a release publishes, so each publishing repo and its deployment check one shape.

• format, version, tag and commit are required. images, sandboxImages, migrations and configSchema are optional, so a release that ships only binaries also fits.
• The top level and every section are closed. A new section is a change to this package.
• The schema uses pattern, not format, so every draft 2020-12 validator gives the same result.
• The package is new, so its first npm publish is manual (scripts/first-publish.sh release-manifest).

Acceptance evidence

• Schema and tests: packages/release-manifest/release.schema.json. 13 tests run in strict Ajv. 4 accept valid manifests, and 9 reject malformed ones (bun test packages/release-manifest: 13 pass).
• Current manifests:

Repo	Latest release	release.json	Result
zgeoff/nixie	v0.28.0	published (sha256 d1c1476f…77db6)	valid
zgeoff/impz	v2.0.3	not yet published (0 of 77 releases)	checked on adoption
zgeoff/auto-mode	auto-mode-v0.21.0	not yet published (0 of 28 releases)	checked on adoption
zgeoff/atc	@zgeoff/atc@4.10.0	not yet published (0 of the latest 100 releases)	checked on adoption

Review

• Local review: codex review --base main reported no findings.
• CodeRabbit: rate-limited, no review.

Testing

• bun run typecheck passes (package tsc, CI pr / checks green)
• bun run test passes (package tests, plus the pre-push hook)
• bun run lint passes (pre-commit hook)
• New tests added for new functionality

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Review configuration:

⚙️ Run configuration

• Configuration used: Repository: zgeoff/coderabbit/.coderabbit.yaml
• Review profile: ASSERTIVE
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• Run ID: 56e0f053-fcb4-4962-94ff-59411c698b3d

📥 Commits

Reviewing files that changed from the base of the PR and between b34db06 and 894d4b7.

⛔ Files ignored due to path filters: 1

• bun.lock is excluded by !**/*.lock, !bun.lock

📒 Files selected for processing: 8

• package.json
• packages/release-manifest/LICENSE
• packages/release-manifest/README.md
• packages/release-manifest/package.json
• packages/release-manifest/release.schema.json
• packages/release-manifest/release.schema.test.ts
• packages/release-manifest/tsconfig.json
• release-please-config.json

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