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 when present
• 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

Applies plugin version discipline by keeping the committed plugin.json version at the 0.0.0 development sentinel, removing the obsolete sync workflow, and adding post-build manifest validation against the release tag.

Changes:

• Reset plugin.json version to 0.0.0 sentinel.
• Delete sync-plugin-version.yml workflow.
• Add validate-contract --for-publish --tag step in release workflow.

Reviewed changes

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

File	Description
plugin.json	Version set back to 0.0.0 development sentinel
.github/workflows/sync-plugin-version.yml	Removed obsolete auto-sync workflow
.github/workflows/release.yml	Added post-build validate-contract step against the tag

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