Use schema-backed domain errors

[kitlangton]

Summary

• Convert core domain errors to Schema.TaggedErrorClass while preserving positional constructors and messages.
• Keep existing _tag matching behavior with schema-backed structured fields.

Verification

• bun run typecheck
• bun run test
• bun src/cli.ts --help
• bun src/cli.ts sync --help
• bun src/cli.ts merge --help

Stack

1. Make replay cleanup interruption-safe #1
2. Use Effect clock for stack timestamps #2
3. Use schema-backed domain errors #3 👈 current
4. Tighten typed error channels #4
5. Configure GitHub polling and concurrency #5