feat: support custom tokenizer modules in the CLI

[Gambit-Checkmate]

What & why

Closes #6: add --tokenizer <module> so CLI analysis and compaction can use the existing custom-counter API instead of always using the built-in estimate.

Changes

• Load a caller-relative or absolute local module and pass its named count(text) export to both CLI modes, leaving the default counter unchanged.
• Report missing flag values, module-load errors, and missing/non-callable count exports without a stack trace.
• Document a gpt-tokenizer adapter, trusted-module execution, and the remaining message-framing/image/protected-turn limitations without adding a runtime dependency.
• Add tests for custom counts, actual budget-driven message removal, path handling, default behavior, and invalid modules.

Validation

The tokenizer integration/error tests were added first and failed on the original CLI, then passed after implementation.

• node --test: 41 passed, 0 failed (Node 24.15.0, Windows).
• node --check bin/tokencut.mjs: passed.
• git diff --check: passed.

Checklist

• Focused change with a clear description
• Ran project tests and syntax checks
• Updated docs and CHANGELOG
• Linked the related issue

AI assistance was used to implement and validate this contribution.