chore: apply plugin version discipline

[intel352]

Part of GoCodeAlone/workflow#760.

Changes:

• keep committed plugin.json.version at the 0.0.0 development sentinel
• remove the obsolete sync-plugin-version workflow
• add post-build release validation so the shipped manifest is checked against the tag

Verification:

• wfctl plugin validate-contract .
• wfctl plugin validate-contract --for-publish --tag v9.9.9 .
• GOWORK=off go test ./...

[Copilot]

Pull request overview

This PR enforces plugin version discipline by keeping the committed plugin.json at the 0.0.0 development sentinel, removing the now-obsolete tag-sync workflow, and adding a post-build validation step that verifies the shipped manifest matches the release tag.

Changes:

• Reset plugin.json version from 0.1.2 to the 0.0.0 development sentinel.
• Delete the sync-plugin-version.yml workflow that previously opened PRs to bump plugin.json on tag pushes.
• Add a post-build wfctl plugin validate-contract --for-publish --tag ... step in release.yml.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
plugin.json	Resets version to 0.0.0 sentinel so the committed manifest no longer claims a release version.
.github/workflows/sync-plugin-version.yml	Removes the obsolete workflow that auto-synced plugin.json from release tags.
.github/workflows/release.yml	Adds post-build validation that the shipped manifest carries the release tag.

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