feat(shell): add here documents infrastructure (Phase 6 M9 partial)

- Add Token variants: HereDoc, HereDocDash, HereString
- Add Redirection variants for here docs and here strings
- Tokenizer recognizes <<, <<-, <<<
- Basic temp file handling for here doc content
- All tests passing (107 unit + 27 integration + 19 property)

TODO for M9 completion:
- Multi-line REPL input reading
- Delimiter detection and validation
- Proper expansion logic (quoted vs unquoted)
- Tab stripping for <<-
- Comprehensive temp file cleanup

Version: 0.12.0 → 0.13.0
Ref: docs/PHASE6_M9_DESIGN.md

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: hyperpolymath