feat: add crab tax plugin

[MrFlounder]

Why

Crabcode already had plugin-based workflows for promptfoo and Excalidraw, but there was no tax workflow for organizing personal tax documents into a reviewable filing handoff. This adds the new tax plugin, wires it into crab, and documents it where plugin-specific users will actually look.

What

• add the crab tax plugin with inventory, extraction, reconciliation, deterministic estimate, and TurboTax handoff generation
• wire install, uninstall, update, help, and execution commands into src/crabcode
• add top-level docs for crab tax and a plugin-local README at plugins/tax/README.md
• add fixture-based tests and testdata for supported flows and failure cases
• add plugin-local .gitignore so build artifacts and dependencies stay out of the repo

How to Test

• run cd plugins/tax && npm test
• run cd plugins/tax && npm run build
• run bash -n src/crabcode
• note: pnpm exec biome check --write . is not runnable in this repo because there is no root package/workspace for pnpm exec to target