[Architecture] oclite: Adopt append-only output with inline status patterns

[randomm]

Context

Research into AIChat and Aider revealed both use an append-only output model with inline status indicators. No fixed panels, no scroll regions — just print to stdout and let the terminal scroll.

This issue tracks adopting those proven patterns after reverting the scroll region approach (#72).

Patterns to Adopt

From AIChat (Rust)

• Cursor position tracking: Use equivalent of crossterm::cursor::position() to know terminal position
• Explicit scroll: ScrollUp/ScrollDown commands when needed (not DECSTBM regions)
• Atomic abort signals: Coordinate Ctrl+C across async operations
• REPL loop: Simple read → process → output → repeat cycle

From Aider (Python)

• Streaming markdown: Full re-render at 8fps (alternative to our incremental approach if edge cases pile up)
• Visual separators: Horizontal rules between messages for readability
• Inline status: Print "Thinking..." or spinner inline, no fixed status bar
• Alt+Enter for multi-line: Toggle between single-line submit and multi-line editing

From blessed (reference only)

• CSR reset recipe (for future use if we ever need fixed panels):
  1. Set CSR: \x1b[{top};{bottom}r
  2. Position cursor inside bounds
  3. Insert/delete lines
  4. ALWAYS reset CSR: \x1b[1;{height}r
  5. Reposition cursor

Implementation

• Ensure output goes through simple process.stdout.write() — no layout abstraction
• Spinner renders inline (before streaming starts, cleared when first chunk arrives)
• Tool calls render inline with compact format (already done: ◇ bash git status → ✓ bash)
• Add horizontal rule separators between messages
• Status info (tokens, elapsed time) renders inline after response completes
• Input naturally at bottom (cursor ends up there after output)

Depends On

• [Architecture] oclite: Revert scroll region layout, adopt append-only output model #73 (revert scroll regions)

Acceptance Criteria

• Clean append-only output flow
• No ANSI scroll region sequences in codebase
• Spinner, tools, markdown all render inline
• Horizontal rules between messages
• oclite feels responsive and clean

[randomm]

Superseded by Live Block architecture (85c396e)