Screen reader support (Plain text output flag to disable TUI dividers)

**What problem would this solve?**
Currently, the Goose CLI relies heavily on ASCII and Unicode TUI formatting elements—specifically stippled and dashed divider lines (`╌⌌╍`)—to structure session output visually.

While visually neat, these characters create major accessibility barriers for visually impaired developers using screen readers like macOS VoiceOver. When a screen reader encounters these long chains of repetitive characters, it reads every single dash or symbol aloud (e.g., repeating "box drawing light horizontal" or "dashed line" dozens of times per interaction). This heavily disrupts the audio reading flow, clutters screen reader speech output, and makes navigating multi-step LLM responses or tool outputs extremely tedious.

**What would a good outcome look like?**
A clean, plain-text output mode where all decorative TUI elements, spinners, and stippled border lines can be bypassed or disabled. Screen reader users would receive a stream of clean, linear Markdown or standard text directly to `stdout`. Interactive prompts and decision choices would be presented as clean, numbered lists (e.g., `1. Option A`, `2. Option B`) that are easy to hear, navigate, and respond to without visual character noise.

**Possible approaches**
* **Command-line flag:** Introduce a flag such as `--plain-text`, `--no-tui`, or `--accessible`.
* **Environment variables:** Automatically disable visual decorations if standard environment variables like `NO_COLOR=1` or `ACCESSIBLE_OUTPUT=1` are present.
* **Stream mode:** Fall back to a standard linear text stream when running in non-interactive/plain terminal modes.

**Additional context**
- **Assistive Technology:** macOS VoiceOver (Screen Reader)
- **Environment:** macOS Terminal / CLI
- The issue affects all CLI interactions where tool output, thought steps, or choices are visually bounded by dashed lines.

- [x] I have verified this does not duplicate an existing feature request
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