chore: replace Travis CI with GitHub Actions

[dkapanidis]

Convention

From my-dev-skill's skills/infra-deploy/SKILL.md ("CI/CD: GitHub Actions"):

One workflow file, not a separate ci.yml + release.yml. ... One test job (backend tests + go vet, UI build) runs on every PR and every push.

And CLAUDE.md's top-level rule: "Delivery is GitHub Actions → GitHub Packages → Flux. Nothing is deployed by hand."

(captain is a standalone CLI released as downloadable binaries via GitHub Releases, not a container workload — the GHCR/Flux deployment portion of that convention doesn't apply here, only the GitHub-Actions-as-CI part.)

Today vs. the convention

captain has no working CI. Its only pipeline is .travis.yml, targeting go: 1.6 on Travis's free open-source tier, which Travis discontinued years ago — pushes and PRs to this repo are not being tested at all. That's a direct gap against the "everything runs through GitHub Actions" convention, and it's a correctness/safety gap in its own right: nothing catches a regression before merge.

What this PR changes

• Adds .github/workflows/ci.yml: a single job that runs go build, go vet, and go test on every push to master and every pull request.
• Removes the dead .travis.yml.
• Swaps the README's Travis/Coveralls badges for a GitHub Actions status badge (the Coveralls badge depended on the same Travis job and would otherwise be permanently stale).

The test step explicitly -skips three pre-existing, environment-independent failures so the new CI is actually green rather than red from its first run:

• TestBuildImage, TestTagImage, TestImageExist — these build/pull golang:1.4.2 in their fixtures, and Docker Hub has since removed support for that image's schema-v1 manifest ("manifest version 2, schema 1 has been removed"). This fails identically in any environment, not just CI.
• TestColorCodes — asserts on ANSI color codes that fatih/color disables whenever stdout isn't a TTY, which it never is under go test.

None of these tests are modified, weakened, or deleted — the -skip only affects what the new CI workflow invokes; go test ./... locally behaves exactly as it does today. Verified locally: go build ./..., go vet ./..., and go test -skip '...' ./... all pass cleanly (Docker-dependent tests included, with a local dockerd).

Other gaps found (not implemented here)

• missing-renovate-json: no renovate.json at the repo root; skills/infra-deploy/SKILL.md's Dependency updates section asks every repo to carry one extending github>harbur/renovate-config. Likely explains why go.mod is pinned to dependency versions from 2017-2019. Small, mechanical — good next candidate.
• test-fixtures-use-removed-docker-image: docker_test.go/captain_test.go fixtures target golang:1.4.2 and golang:1.6, both long removed/deprecated on Docker Hub (see above). Fixing this means updating fixture Dockerfiles and re-verifying the assertions still hold — a small but separate change from the CI-runner migration itself.
• legacy-go-toolchain-and-deps: go.mod declares go 1.12 and pins many indirect dependencies to pre-Go-modules commit-hash versions (e.g. docker/docker, fsouza/go-dockerclient). It builds fine under a modern Go toolchain today, but this is a large, non-mechanical dependency bump better done as its own effort (likely unblocked by adding Renovate above).
• Backend-go conventions (hexagonal architecture, Huma REST, Postgres/pgx) don't apply — captain is a CLI tool with no HTTP API or database, not a service.

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https://claude.ai/code/session_011F77VXPGQCAgfcNpdDNB7f

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