feat: add tui.showThinking config toggle for reasoning visibility

[avoidwork]

Summary

Add a tui.showThinking config toggle to conditionally render the reasoning/thinking element in MessageBubble.

Motivation

The TUI has plumbing for reasoning content (reasoningContent field on the message model, rendered in MessageBubble at line 157-173), but it is always rendered when present. Users may want to hide thinking text for cleaner output. The config toggle provides control without requiring graph-level changes.

Proposed Solution

1. Add tui.showThinking: true (default: true for backward compat) to config schema
2. In messageBubble.js, check the config before rendering reasoningEl:

   const showReasoning = config?.tui?.showThinking !== false;
   const reasoningEl = showReasoning && hasReasoning ? ... : null;

3. Add config validation in src/config/schemas.js

Alternatives Considered

• Hardcoding the toggle off — less flexible, no user control
• Stripping reasoning before it hits the TUI — loses the data entirely, config toggle preserves it

OpenSpec Note

This project uses OpenSpec for feature development. If this request is approved, I will:

1. Run /opsx:propose to generate a full proposal with specs and tasks
2. Iterate on the design before any code is written
3. Follow the task-driven implementation workflow

Additional Context

• Message model already has reasoningContent — no model changes needed
• reasoningEl at messageBubble.js:161-173 is already conditionally rendered based on hasReasoning — just needs a config gate
• committedReasoning in app.js is always "" — reasoning content is never populated, but the rendering path exists for when it is

Audit Findings (for Issue #600)

• src/tui/messageBubble.js:157 — hasReasoning check: role === "assistant" && reasoningContent
• src/tui/messageBubble.js:161-173 — reasoningEl renders (thinking) ... truncated at 200 chars when hasReasoning is true
• src/tui/messageBubble.js:243 — reasoningEl rendered in the bubble JSX
• src/config/schemas.js — Need to add tui.showThinking to the TUI config schema
• src/tui/app.js:717 — reasoningContent: committedReasoning || undefined — ready to receive reasoning data

Implementation scope: One config field, one conditional in MessageBubble, one config schema update. Low risk, straightforward.