DRY: extract shared error-handling helpers

[clockblocker]

Summary

• Added extractErrorReason() and toApiCommandError() to errors.ts — centralizes the repeated "reason" in error ? ... ternary (4 occurrences) and ResultAsync.fromPromise error conversion (2 occurrences)
• Added notifyAndLogError() in orchestration/shared/notify-error.ts — replaces 3 identical .mapErr() blocks that notify the user and log the error
• Net result: 6 files changed, 42 insertions, 43 deletions (removed more code than added)

Files changed

File	Change
errors.ts	Added extractErrorReason, toApiCommandError helpers
orchestration/shared/notify-error.ts	New file: notifyAndLogError helper
textfresser.ts	Use notifyAndLogError instead of inline .mapErr()
orchestration/lemma/execute-lemma-flow.ts	Use notifyAndLogError in both error paths
orchestration/background/background-generate-coordinator.ts	Use extractErrorReason
commands/generate/steps/generate-sections.ts	Use toApiCommandError for both ResultAsync.fromPromise calls

Test plan

• TypeScript type check passes (no new errors)
• All 1150 unit tests pass (6 pre-existing failures unchanged)
• Biome lint passes on all modified files
• Pure structural refactor — no behavior changes

🤖 Generated with Claude Code