fix(telegram): guard against empty renders and fix misleading example (#26)

[fxthiry]

Summary

Fixes #26. When a user copies the example config pattern (title: \"{{ title | default('Alert') }}\" / body: \"{{ body }}\"), both templates resolve to empty strings at render time because VictoriaLogs events do not carry title / body fields. Telegram's parse_mode: HTML default then receives a near-empty payload (<b></b>\n) and responds HTTP 400 Bad Request. The alert is silently lost.

Two coupled fixes:

• Runtime guard in TelegramNotifier::prepare_text. If the rendered text is empty, whitespace-only, or contains no text after stripping HTML tags, substitute a priority-ordered fallback and emit a structured warn!. Priority: <b>{title}</b> if title non-empty, else Alert: {rule_name}, else (valerter alert, empty render). Title and rule_name are HTML-escaped and the title is capped to stay under Telegram's 4096 codepoint limit once wrapped.

• Honest example in config/config.example.yaml. New comment block separates "variables available inside the template" (VL event fields, rule_name, log_timestamp*) from "template output keys" (title, body, body_html, accent_color). The default_alert entry switches to title: "{{ rule_name }}" and body: "{{ _msg }}" so it renders non-empty content out of the box on any VL event.

Test plan

• cargo test --lib — 429 passed (baseline 415 + 14 new: 11 from initial TDD + 3 from review patches)
• cargo clippy --all-targets --all-features -- -D warnings — clean
• cargo run --bin valerter -- --validate -c config/config.example.yaml — exit 0
• Smoke-test live once merged with fix(templates): accept dotted VictoriaLogs fields in templates (#25) #27 on release/1.1.1 (prod-test-telegram config + broken template repro)

Review hints

Adversarial review flagged three issues that are patched in this PR (HTML injection via title/rule_name in the fallback, truncate breaking mid-tag on long titles, em-dash in a user-facing string). See _bmad-output/implementation-artifacts/deferred-work.md for seven follow-up items explicitly deferred.

Targeted at release/1.1.1 so it can be bundled with #27 and validated by the reporter before tagging v1.1.1.