refactor(workflow): model operational failures with better-result

[Waishnav]

Summary

• Add a Better Result error taxonomy for workflow files, store transitions, worktrees, providers, schema enforcement, and corrupt persisted data.
• Introduce Result-returning operational cores while preserving throwing compatibility wrappers and the ergonomic workflow-script agent() API.
• Normalize provider failures at adapter boundaries; native schema fallback now branches on a typed unsupported-schema error rather than arbitrary exceptions.
• Map the same tagged errors exhaustively to journal error kinds, CLI exit codes, and structured MCP tool errors.
• Preserve primary worktree failures, attach cleanup failures, and stop silently degrading corrupt structured replay data to text.

Stack

• Base: pr/dw-7-typed-contracts-zod (PR refactor(workflow): strengthen contracts and zod boundaries #100)
• Branch: pr/dw-8-result-errors

Test plan

• npm run typecheck
• npm test
• npm run build
• Result-path tests for files, providers, store transitions, schema exhaustion, worktrees, CLI/MCP serialization
• Regression test: worktree setup failure is not replaced by a missing journal-call error
• git diff --check

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