feat(telegram): render restrained emphasis with safe text entities

[hegelstad]

Summary

• Render a deliberately small outgoing Telegram syntax: **bold**, inline code, and fenced code blocks with optional language labels.
• Send native text entities rather than parse_mode, keeping shell operators, paths, HTML and other literal text safe from Telegram escaping rules.
• Paginate with UTF-16-aware lengths and UTF-8-safe cuts; clip/rebase entities across pages and leave the existing three-page truncation notice unstyled.
• Preserve paragraphs/lists, document unsupported syntax, and add unit, HTTP boundary, Unicode/pagination and fuzz tests. No extra dependency or Telegram SDK.

Scope

This is an independent follow-up to #29, based directly on main, with no image changes included. Image captions, stored messages, and other bridges remain unchanged. No terminal color emulation or full Markdown renderer is introduced.

Validation

• This main-based branch: make check and go test ./internal/bridge/telegram ./internal/bridgeservice -count=1 passed.
• Equivalent formatting change combined with feat(telegram): send and receive JPEG/PNG images #29: make check, full make test, fresh go test ./tests/external -count=1, targeted transport -race, and a short two-worker renderer fuzz run passed; binary built successfully.
• Combined build activated with operator approval; live Telegram API accepted a message containing bold, inline code and a fenced code block. Client-side visual acceptance is still pending operator feedback.

The image PR uses context-aware HTTP for sendMessage; this standalone branch intentionally preserves main's existing HTTP call to keep the two changes independent. A merge of both will require reconciling that small sendChunk hunk.