chore: drop yarn zero-install

[ndthanhdev]

Summary

Moves the repo off Yarn zero-install. The offline cache is no longer committed; Yarn now uses the shared global cache and CI restores dependencies via actions/setup-node.

Also includes a drive-by fix to the Dagger build env, which was broken independently of this change (see below).

Changes

Dropping zero-install (1084e13)

• .gitignore — removed the !.yarn/cache exception, so the cache is ignored. patches/plugins/releases/sdks/versions stay tracked.
• .yarnrc.yml — enableGlobalCache: true, removed compressionLevel: mixed.
• Untracked .yarn/cache (1,828 zips, 175MB) and the generated .yarn/install-state.gz, which had been force-added despite matching .yarn/*.
• yarn.lock — dropping compressionLevel: mixed moves the lockfile to Yarn 4's default cacheKey (10 → 10c0), rehashing all 1,820 checksums. No resolution or version changed.
• All 5 workflows — added cache: yarn to actions/setup-node and hardened installs to yarn install --immutable so lockfile drift fails the build.

Unrelated CI fix (61bd2bb)

workflow_runtime.go:25 pinned proto to 0.58.1, but the Node.js proto plugin is resolved unpinned from the registry at container build time and has since published a release requiring >= 0.60.0-alpha.0. proto use therefore failed on any run, regardless of this PR. Bumped to 0.60.2 (latest stable) in both the real build env and the skeleton template, and corrected the stale comment that still said 0.35.3.

Verification

• Cold rm -rf node_modules && yarn install succeeded locally from an empty cache.
• yarn install --immutable then passed with a clean working tree.
• Parsed all 5 workflow YAMLs and asserted cache: yarn + yarn install --immutable on each.
• Workspace Verify is green on CI, including a cold registry install with no committed cache — the key thing this PR needed to prove.

Notes for reviewers

• The 1,835-file diff is almost entirely cache deletions and lockfile checksum churn. The substantive change is a handful of files: .gitignore, .yarnrc.yml, the workflows, and the proto pin.
• Repo size is unchanged (~865MB). The historical zips still live in the pack; only a history rewrite would reclaim that, which was deliberately out of scope.
• After merging, pull and run yarn install — packages repopulate from the registry once, into the global cache.
• First CI run after merge is a cold install; subsequent runs hit the yarn.lock-keyed cache.