fix: use dynamic bash lookup and stdin message passing for Telegram scripts

[suselee]

Problem

When the agent sends Telegram messages via bash tool, the message content is embedded directly in shell command arguments. This causes two issues:

1. Shell escaping failures: Special characters (', ", $, !, backticks) in message content are interpreted by the shell, resulting in truncated messages, variable substitution, or syntax errors like Syntax error: word unexpected (expecting ")").

2. Hardcoded /bin/bash path: shell_manager.py assumed bash lives at /bin/bash, which fails on FreeBSD where bash is installed at /usr/local/bin/bash.

The core problem is architectural: arbitrary text content should never pass through shell interpretation. This is especially problematic with smaller models (e.g. Gemma 31B) that tend to add redundant /bin/sh -c '...' wrappers, creating nested shell layers that make correct escaping nearly impossible.

Changes

src/bub/builtin/shell_manager.py

• Replace hardcoded /bin/bash with shutil.which("bash") or shutil.which("sh") for cross-platform compatibility (Linux, FreeBSD, macOS).

src/skills/telegram/scripts/telegram_send.py

• Support --message - to read message content from stdin, bypassing shell interpretation entirely.
• Backward compatible — --message 'text' still works as before.

src/skills/telegram/scripts/telegram_edit.py

• Support --text - to read message content from stdin (same approach as telegram_send.py).

src/skills/telegram/SKILL.md

• Update all command templates to use heredoc stdin piping (cat << 'EOF' | ... --message -).
• Add explicit execution policy rule: always pass message content via stdin, never inline in shell arguments.
• This lowers the barrier for smaller models that struggle with shell escaping, while remaining fully compatible with stronger models.

Test plan

• uv run ruff check . passes
• uv run pytest -q — all tests pass
• Verified on FreeBSD jail: shutil.which("bash") correctly resolves /usr/local/bin/bash
• Backward compatible — existing --message 'text' / --text 'text' usage still works
• Deployed and tested on FreeBSD jail with Telegram gateway

[frostming]

lgtm