ci: add GitHub Actions workflow (build gate + advisory vet/gofmt/race)

[Dirtybird99]

What

Adds the repository's first CI workflow (.github/workflows/ci.yml). It runs on pushes to community-dev / community-mainnet / community-testnet / main and on all pull requests.

Two jobs:

• build (blocking) — go build ./.... This is green on the current tree, so it's safe to make a required status check and will catch compile breakage in future PRs.
• quality (advisory) — go vet ./..., a gofmt check over non-vendor sources, and go test -race ./walletapi/.... Every step is continue-on-error, so the check stays green while the pre-existing findings are burned down; the output still shows in the logs. Drop continue-on-error from a step once its debt reaches zero to promote it to a gate.

Why

The repo currently has no .github/workflows at all. A build gate alone would have caught the concurrent-map crash class discussed in #26, and the race step surfaces the walletapi data races filed as #33–#37.

Verification done before opening this PR

Against community-dev @ 6f9bd2c:

Check	Result
go build ./... (Windows, go 1.26)	✅ exit 0
GOOS=linux GOARCH=amd64 go build ./... (cross-compile; project has no cgo)	✅ exit 0
go vet ./...	❌ exit 1 (unreachable-code / unkeyed-struct warnings) — advisory
gofmt -l (non-vendor tracked .go)	260 files — advisory
go test -race ./walletapi/...	❌ real data race in sync_loop (daemon_communication.go:217) + xswd rate-limit test — advisory, tracked in #33/#34

So the blocking job is verified green on the Linux target; the three currently-red checks are intentionally advisory and each links to tracked debt.

Suggested adoption path

1. Merge, then mark build as a required status check in branch protection for community-dev.
2. Burn down the advisory findings (start with the races in walletapi: SyncHistory mutates EntriesNative without w.Lock() — reader RLocks do not exclude the writer #33–walletapi: concurrent Reset on shared watchdog time.Timer #37), then flip each step to blocking by removing its continue-on-error.

Note

Because this is a first-time contribution, a maintainer may need to approve the workflow run before it executes on this PR. The workflow file itself is minimal and uses only actions/checkout and actions/setup-go.