fix: use [[]] delimiters for placeholders to prevent HTML encoding issues (#38)

fix: use [[]] delimiters for placeholders to prevent HTML encoding issues (#38)

Fixes #36 - HTML-encoded placeholders now unmask correctly

Changes:
- Changed placeholder format from <TYPE_N> to [[TYPE_N]]
- Created src/constants/placeholders.ts as single source of truth
- Removed configurable redact_placeholder (was bug - streaming hardcoded [[)
- Updated dashboard regex for yellow highlighting
- Added tests for HTML/JSON/URL contexts

The [[]] delimiters are safe in HTML, JSON, and URLs - they don't get
entity-encoded like <> did.