Add CONTRIBUTING guide and surface contributor onboarding from README

[Copilot]

This PR adds a minimal contributor guide so first-time contributors can find setup, contribution policy, and pre-PR expectations without digging through README and CI config. It also adds an explicit README entry point to reduce onboarding friction.

• Contributor entry point

  • Add root-level CONTRIBUTING.md
  • Link to it from both README.md and README.en.md

• Contribution policy

  • Clarify that bundled core vocabulary additions are not accepted as PRs
  • Call out that fixes to already-registered vocabulary data are welcome
  • Set expectation that maintainer response time may vary

• Development setup

  • Document required tooling and versions for normal development vs. dictionary pipeline work
  • Include clone / initial setup commands
  • Document how to enable lefthook

• PR workflow expectations

  • Define when contributors should open an issue first
  • Clarify that small docs / typo / narrow fixes can go straight to PR
  • Set expectations for branch naming, commit scope, and PR descriptions

• Quality gates

  • Centralize the minimum pre-PR commands for format, lint, tests, and build
  • Call out the extra shellcheck requirement when install.sh changes

• Change-specific review notes

  • Add short guidance for TUI changes
  • Add cautions for dictionary/data changes and license-sensitive files
  • Add reminders for release/distribution-related edits

Example of the documented local quality checks:

goimports -w cmd internal assets
golangci-lint run
go test ./...
go build ./...

---

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

_{TIP This summary will be updated as you push new changes. Give us feedback}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.