Require exact special command types

[claell]

Summary

• Dispatch comment, preamble, and string as special commands only on an
  exact case-insensitive type match.
• Preserve longer custom types such as commentary, preamble_study, and
  string_theory as ordinary entries.

Fixes #568.

Validation

• Focused custom-entry regression matrix under warnings-as-errors: 3 passed.
• Complete upstream suite: 2,579 passed, 12 skipped, with the four existing
  deprecation warnings in tests/test_entrypoint.py.
• git diff --check: passed.
• The project pre-commit executable was not available in the isolated local
  environment; GitHub CI should independently run the configured hooks.

Review note

This change was developed and validated in a concentrated session rather than
exercised over a long period in production. Although the dispatch change is
small and covered across all three prefixes, careful human review is requested.

AI assistance

This pull request was prepared with ChatGPT Codex using GPT-5.6 Sol with high
reasoning effort. Codex assisted with analysis, implementation, branch
isolation, and test execution. Automated validation is not a substitute for
maintainer review.