feat: add vexdo init command and OSS polish (docs, CI, tests)

[ubcent]

Motivation

• Provide an interactive bootstrap for projects so users can run vexdo init to quickly create a working .vexdo.yml, task lanes, and logs, and ensure .vexdo/ is ignored.
• Ship repository polish required for publishing: CI workflow, full README, CONTRIBUTING guide, and a Claude-specific guide.

Description

• Added an interactive vexdo init command (src/commands/init.ts) that prompts (readline) for services, per-service paths, review model, max iterations, auto-submit, and codex model, writes .vexdo.yml, creates tasks/{backlog,in_progress,review,done,blocked}, creates .vexdo/logs/, and appends .vexdo/ to .gitignore without duplicates.
• Registered the init command in the CLI (src/index.ts) and added unit tests (tests/unit/init.test.ts) covering config generation, directory creation, gitignore handling, and overwrite prompt behavior.
• Added GitHub Actions CI workflow (.github/workflows/ci.yml) with lint-typecheck, test, build, and tagged release jobs (npm publish), including actions/cache for npm.
• Rewrote README.md with usage, flow diagram, requirements, quick start, command docs, task and config formats, spec guide, troubleshooting, and CONTRIBUTING link.
• Added CONTRIBUTING.md and CLAUDE.md with contributor and agent guidance.
• Added test:coverage script and updated package.json scripts accordingly.

Testing

• Ran npm run typecheck and it completed successfully.
• Ran npm test / npm run test:coverage and all tests passed (unit + integration), including the new runInit unit tests.
• Ran npm run build and the build produced dist/ artifacts successfully.
• Ran npm run lint and it reported existing repository lint errors unrelated to the new init implementation; this PR does not attempt a full lint cleanup.

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Codex Task