docs: add TUI blueprint for next-generation terminal interface

[avoidwork]

Description

Adds a comprehensive design blueprint for a planned TUI rewrite. The document describes a clean-slate terminal interface grounded in the existing implementation patterns (Ink + ink-scroll-view + structured logger) and inspired by the best patterns of the bitchx IRC client.

This is a design spec — not a description of the current TUI. It defines the philosophy, architecture, scrolling model, message display, runtime configuration, command parser, streaming pipeline, session lifecycle, and proposed improvements for a future implementation.

Key sections:

• Philosophy — input is primary, output is a log, silence is the default
• Architecture — component hierarchy, dependencies (Ink, ink-scroll-view, pino)
• Scrolling & Viewport — ink-scroll-view API, auto-scroll behavior, keyboard navigation
• Message Display — role-based styling, memoization, markdown rendering, tool call display
• Runtime Configuration — bitchx-inspired toggle commands (/toggle, /format, /level)
• Command Parser — dispatch table, skill execution, registered commands
• Streaming Architecture — full data flow from user input through dispatchProvider to UI updates
• Session & Persistence — session lifecycle, context token calculation, GC integration
• Onboarding & Banner — first-run experience
• Architectural Debt — honest assessment of current code (useState vs useReducer, inline streaming, flat file structure)
• Proposed Implementation — TUIState interface, action types, command registry schema, streaming callback transformation

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

N/A — documentation-only change.

Coverage

• 100% line coverage maintained
  N/A — no code changes.

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied