added professional tone - #906

Description

• before enabling Professional Tone

• after enabling Professional Tone

Adds a professionalTone preference that switches Nanocoder to a strictly
functional, "boring" output style. Closes #896.

When enabled:

• Progress text drops its random adjective — the end-of-turn note reads
  Completed in 12s. instead of Worked for a plucky 12s.
• System prompt gains a ## TONE section instructing the model to stay
  terse: no preamble ("Sure!", "Let me..."), no narration of what it is about
  to do, no celebratory wrap-ups, no emoji, facts only.

Toggle lives at /settings → Behavior → Professional Tone, and
persists to nanocoder-preferences.json as "professionalTone": true.

Two notes for reviewers:

1. The completion note was the only whimsical progress string in the codebase —
   every spinner label (Loading..., Executing tool:, Cancelling...) is
   already plain, so the issue's "simplify the progress spinner text" had
   nothing else to act on.
2. The prompt section applies on the next prompt rebuild (mode switch, model
   switch, or restart), not mid-session, because useChatHandler caches the
   base prompt for KV-cache stability. This matches how reasoningExpanded and
   alternateScreen already behave when changed from the settings panel, and
   is documented in docs/configuration/preferences.md.

Files

• source/types/config.ts — professionalTone?: boolean on UserPreferences
• source/config/preferences.ts — getProfessionalTone / updateProfessionalTone
• source/utils/completion-note.ts — buildCompletionNote, wired into conversation-loop.tsx
• source/app/prompts/sections/professional-tone.md — the TONE section
• source/utils/prompt-builder.ts — gates the section on the preference
• source/app/components/settings-tabs.tsx — Behavior-tab toggle
• docs/configuration/preferences.md — documented

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

8 new tests across preferences.spec.ts (persistence, default-when-unset,
preserving unrelated keys), completion-note.spec.ts (both phrasings, minute
formatting), prompt-builder.spec.ts (section present/absent, ordering before
the system-info block), and settings-tabs.spec.tsx (row renders on the
Behavior tab).

test:all does not complete on this branch — but it does not complete on a
clean main either. I ran the full AVA suite both ways and diffed the failure
lists:

test:format, test:types, test:lint and test:knip all pass.

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Manually verified with Google Gemini (gemini-2.5-flash): toggled the
setting in /settings → Behavior, confirmed "professionalTone": true
persisted to the preferences file, and confirmed the completion note switches
phrasing. Also verified the prompt wiring directly:

node -e "import('./dist/utils/prompt-builder.js').then(m=>console.log(m.buildSystemPrompt('normal',undefined,['read_file']).includes('## TONE')))"

which prints true with the preference on and false with it off.

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging — n/a, no new failure paths