fix(tgapp): polish Telegram MarkdownV2 rendering and startup helpers

[YooooEX]

Context

Telegram replies already use a MarkdownV2 rendering path, but a few edge cases still made the output less
polished:

• [FILE:...] markers were removed from the message body after attachment extraction, so users could receive
  attachments without an inline filename reference.
• Telegram startup still duplicated helper logic that is already shared by other chat frontends.
• Common Markdown styles like __bold__ and ~~strikethrough~~ were not converted into Telegram MarkdownV2
  entities.

This PR keeps the change radius inside frontends/tgapp.py. No new dependencies are added.

Summary

Polish Telegram message rendering and startup wiring by reusing shared frontend helpers, keeping file marker
basenames visible in replies, and expanding Markdown-to-MarkdownV2 conversion for common bold and strikethrough
styles.

Changes

frontends/tgapp.py

• Reuse shared chat frontend helpers for single-instance locking, runtime checks, and log redirection.
• Keep [FILE:...] markers visible in the reply body as basenames while still resolving and sending matching
  files as Telegram attachments.
• Convert common Markdown __bold__ input to Telegram MarkdownV2 bold output.
• Convert GFM-style ~~strikethrough~~ input to Telegram MarkdownV2 strikethrough output.

[lsdefine]

LGTM. Confirmed it applies cleanly and the Telegram polish changes are scoped.