Replace instanceof checks with Effect guards

[K-Mistele]

Summary

• replace runtime instanceof checks with Effect guards such as Predicate.isError, Predicate.isTagged, and AiError.isAiError
• model hook execution failures as Data.TaggedError values and expose a tag-based guard for cause inspection
• use Predicate.hasProperty and Predicate.isString for unknown web error properties, with an explicit Effect dependency in the theme package

Verification

• bun run typecheck
• bun run test
• bun run --cwd packages/fold-codex test
• bun run --cwd packages/fold-core test
• changed-file oxfmt and oxlint checks
• PR-added-line audit confirms no instanceof, typeof, property in, or null checks
• repository audit confirms no executable instanceof usage remains