ci: add v2 prerelease notes and tags

[mynameistito]

Summary

• Add idempotent v2 prerelease staging behavior.
• Extract 2.0.0-next.N notes from CHANGELOG.md, matching stable releases.
• Create a GitHub prerelease tagged v2.0.0-next.N against the exact opencode-v2 commit.
• Support backfilling the GitHub release when the npm version is already published.

This does not move the latest dist-tag.

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Summary by cubic

Adds idempotent v2 prerelease staging on npm and creates matching GitHub prereleases for 2.0.0-next.N with notes from CHANGELOG.md. Previously CI only tried to publish a preview; now it stages or skips if already published and creates a GitHub prerelease, without moving the latest dist-tag.

• Replaces the publish step with an idempotent “Stage v2 preview” that uses npm view and tolerates “already staged/published” errors; keeps the next dist-tag.
• Extracts release notes from the ## <version> section of CHANGELOG.md; falls back to a generic message if missing.
• Creates a GitHub prerelease v<version> targeting the exact opencode-v2 commit only when it does not already exist; safe to rerun and to backfill after an npm publish.
• Sets PACKAGE_NAME to oc-usage-limits-plugin.

^{Written for commit 5b6721e. Summary will update on new commits.}

[changeset-bot]

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Latest commit: 5b6721e

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