Fix Telegram formatting: avoid tables, use code blocks for commands

[mattmezza]

Problem

Problem 1: Telegram does not render Markdown or ASCII tables properly. When the agent includes tabular data in a response the result is a garbled wall of text.

Problem 2: Commands and code snippets like run_command: gh issue create --repo ... are sent as plain text, making them hard to distinguish from the surrounding message.

Changes

Prompt-level instructions (all character definitions)

• Added a "Telegram message formatting" section to character.md.example with three rules:
  • Never use tables — present structured data as bullet lists or natural language
  • Always wrap commands and code snippets in inline code backticks or fenced code blocks
  • Headers → use bold instead of # heading syntax
• Added the same formatting guideline to all 6 persona files:
  • personae/coding-helper.md
  • personae/finance-assistant.md
  • personae/fitness-coach.md
  • personae/language-tutor.md
  • personae/travel-planner.md
  • personae/writing-editor.md

Post-processing safety net (channels/markdown_tg.py)

• Added _strip_tables() — a belt-and-suspenders function that detects ASCII/Markdown tables (pipe-delimited rows with a ---|--- separator line) and converts them to bullet lists before rendering
• Called as the first step in to_telegram_html() so tables are neutralised even if the prompt instruction is missed
• All existing tests continue to pass

Tests

• Added new test cases for table conversion, pipe-in-code preservation, and header row handling

Closes #51