Add regression tests for zod 3/4 logger schema + GitHub Actions CI

[bjcoombs]

Summary

Follow-up to #18 (merged as v0.0.3). Two additions that didn't land in time:

1. Regression tests for the logger schema across zod 3 and zod 4, covering the code paths that were broken in v0.0.2 (ai-sdk-v5 tag): z.function().args() incompatible with zod 4 #17.
2. GitHub Actions CI workflow - this repo has no CI today, so issues like v0.0.2 (ai-sdk-v5 tag): z.function().args() incompatible with zod 4 #17 only surface at consumer install time.

Changes

`src/validation.test.ts` - 6 new tests

Exercise the `settings.logger` validation path (previously uncovered) to lock in zod 3/4 compatibility:

• Valid logger with `warn` + `error` accepted
• Valid logger with optional `debug` accepted
• `logger: false` (disable logging) accepted
• Logger with non-function `warn` rejected
• Logger with missing `error` rejected
• Nested logger under `defaultSettings` validated through the provider schema

`.github/workflows/ci.yml`

Minimal CI workflow running on push/PR to `main` and `ai-sdk-v5`:

• Matrix tests against `zod@^3.24.0` and `zod@^4.1.0` (the declared peer range)
• Steps: `npm ci` → swap zod version → typecheck → lint (non-blocking) → build → vitest → import smoke test
• The final step (`require('./dist/index.cjs').createOpencode({})`) is specifically what would have caught v0.0.2 (ai-sdk-v5 tag): z.function().args() incompatible with zod 4 #17

Testing

• `npm test` - 275 tests pass (was 269 pre-Fix zod 4 incompatibility in loggerSchema (closes #17) #18, 269 + 6 new)
• Verified under zod 3 and zod 4 locally
• Workflow is self-contained; will activate on merge

Risk

• None for the tests - pure additions
• CI workflow: no existing workflow to conflict with. Lint step is `continue-on-error: true` to avoid blocking on style issues I haven't audited.

Test plan

• Tests pass under zod 3
• Tests pass under zod 4
• Full suite (275 tests) green
• CI run on this PR validates the matrix