ci: automate v2 releases with Release Please

[draedful]

The v2 branch currently requires manual version bumps and publishing. This change makes ordinary v2 pushes create or update one Release Please manifest PR; merging that version PR validates the exact release commit and packed artifacts before publishing public packages to the next channel in dependency order.

The manifest tracks Graph and the private scheduler, and updates workspace peer dependencies for future public packages. Publication supports recovery from a completed dependency prefix and preserves the v1 latest channel. The release PR remains autorelease: pending until publication, tags, and GitHub releases complete, then transitions to autorelease: tagged.

Regression coverage uses Release Please 17.6.0, matching the pinned action, to exercise the first combined release, repeated PR updates, pending-release blocking, and completion. It also accepts both plain first-release headings and linked later-release headings when extracting changelog entries.

Validation:

• pnpm run lint
• pnpm run test: scheduler 9 passed; Graph 340 passed, 4 skipped, 6 todo; release contract 39 passed
• pnpm run typecheck
• PACKAGE_CONTRACT_TARBALL_PATH=/private/tmp/graph-v2-merge-artifact.tgz pnpm run test:package-contract: exact tarball retained, isolated vanilla and React consumers passed
• Full Playwright suite: 186 passed with a temporary configuration using port 16006 because 6006 was occupied
• pnpm run build-storybook
• pnpm install --frozen-lockfile --offline --ignore-scripts
• Workflow YAML parsing and syntax checks for its Bash and embedded JavaScript
• git diff --check

Related to #333. Hosted release execution and the first npm prerelease will be validated after the infrastructure is merged and its generated release PR is reviewed.

[gravity-ui-bot]

Preview is ready.