feat: enforce tagged error messages

[rekram1-node]

Summary

• add a repository-local Oxlint rule requiring Schema.TaggedErrorClass declarations to expose a message
• migrate operational errors to field-derived message getters and document narrow sentinel exemptions
• add fixture coverage and a dedicated PR lint workflow

Verification

• bun lint:test
• bun lint --quiet
• bun test test/util/error.test.ts test/background-job.test.ts from packages/core
• bun typecheck from packages/core

Stacked on #33530.

[github-actions]

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