Follow-up: unresolved review items from PR #28

[gentlementlegen]

Context

Follow-up ticket for unresolved/actionable feedback from PR #28.

Required changes

• Remove stale commands when TCommand = null: if allowMissingCommandSchema is true and no command schema is found, explicitly delete manifest.commands.
• Align manifest discovery with action inputs: either pass pluginEntry/schemaPath into update-manifest.js and use them, or enforce/document that entrypoint discovery is limited to src/**/*.ts.
• Clean up lint/static-analysis findings in .github/scripts/update-manifest.js:
  • remove unused ENTRYPOINT_FNS;
  • simplify/fix the commandSchema null-check warning path.
• Improve parser maintainability/readability (captured in review): reduce low-level parsing complexity and document why Node+optional Deno remains the chosen runtime (or propose a Deno-first alternative).

Acceptance criteria

• manifest.commands is omitted when TCommand = null, with test coverage.
• Entrypoint contract is consistent between action.yml, update-manifest.js, and README.md.
• npm test passes.