feat(ci): apidiff reusable workflow, enforce compatible

[erikburt]

Changes

• Migrates apidiff-go to the reusable workflow
• Enforces compatibility

Testing

• No-op on no go changes: https://github.com/smartcontractkit/chainlink-common/actions/runs/26161773643/job/76955205589?pr=2075
• Failure on incompatible changes: https://github.com/smartcontractkit/chainlink-common/actions/runs/26165378803/job/76968002244?pr=2076#step:4:101
• Bypass failure when PR is labeled: https://github.com/smartcontractkit/chainlink-common/actions/runs/26165543893/job/76968614722?pr=2076#step:4:99

[Copilot]

Pull request overview

This PR updates the repository’s API-diff CI to use a centralized reusable workflow and changes the behavior to enforce API compatibility (with an opt-out label).

Changes:

• Replaces the in-repo changed-modules + matrix-based apidiff job with a call to smartcontractkit/.github’s reusable apidiff workflow.
• Adds workflow-level permissions and enables compatibility enforcement unless the PR has an allow-incompatible label.

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