ci: modernize deployment — replace Travis with GitHub Actions + add RELEASING.md

[TheRealAgentK]

Summary

Stacked on top of #55 — please merge that one first; then this PR's diff against main will become small and self-contained.

Modernises the deployment story:

1. Replaces Travis CI with GitHub Actions. Travis is effectively dead for OSS projects.
2. Documents the manual npm publish workflow in a new RELEASING.md, since this package is distributed via npm and there's no other host to push to.

No source-code changes — only CI config and docs.

Verification

Locally on this branch:

• ✅ npm run lint — clean
• ✅ npx ember build — succeeds
• ✅ npx ember test — 27/27 passing
• ✅ npm pack --dry-run — same 10 files as before; no CI configs, AGENTS.md, or RELEASING.md leak into the npm tarball
• ✅ js-yaml parses .github/workflows/ci.yml cleanly; jobs: test, floating-deps, try-scenarios

In CI on this PR: all hard-required jobs are green (Lint, Build, Test, Floating Dependencies, and the three pinned-Ember Try: scenarios).

Commits

ci: migrate from Travis CI to GitHub Actions

• Delete .travis.yml (and remove its entry from .npmignore).

• Add .github/workflows/ci.yml with three jobs:

  Job	Triggers	What it does
  test	every PR + push to main/master	npm ci → npm run lint → npm run build → npm run test:ember on Node 20
  floating-deps	every PR + push to main/master	same, but npm install --no-package-lock to catch breakage from latest matching versions
  try-scenarios	runs after test passes	ember-try matrix — see below

  Concurrency is set so a new push to a PR branch cancels the previous in-flight run.

  try-scenarios matrix:

  Scenario	Required?
  ember-lts-4.4.5	✅ hard requirement
  ember-lts-4.8.6	✅ hard requirement
  ember-lts-4.12.3	✅ hard requirement
  ember-release	⚠️ allow-failure: true
  ember-beta	⚠️ allow-failure: true
  ember-canary	⚠️ allow-failure: true

  The three pinned LTS lines are hard-required. release/beta/canary are allow-failure: true because the addon is currently pinned to the Ember 4.12 line; the Ember 5/6 jump is tracked separately and will inevitably break those scenarios until that work lands.

  Removed scenarios (vs. the old config/ember-try.js):

  • ember-classic — the Ember Classic edition was removed from Ember itself, so the scenario can never pass on a modern Ember version. See deprecations.emberjs.com/v3.x/#toc_editions-classic.
  • ember-default-with-jquery — the jquery-integration optional feature was removed in Ember 4.0. See deprecations.emberjs.com/v3.x/#toc_optional-feature-jquery-integration.

  Both are also dropped from config/ember-try.js with explanatory comments.

• Update README.md badge: Travis → GitHub Actions.

• Update AGENTS.md §5 (CI section) and §9 (release section).

• Update .npmignore to exclude .github/, AGENTS.md, RELEASING.md from the published tarball.

docs: add RELEASING.md describing the publish process

New file documenting:

• Prerequisites (npm publish access, npm login, 2FA).
• SemVer guidance for choosing patch/minor/major.
• Step-by-step release commands: npm version → git push --follow-tags → npm publish → GitHub Release.
• What gets published (per .npmignore) and how to preview with npm pack --dry-run.
• Pre-flight checklist.
• Recovery steps if a bad version is published (72-hour npm unpublish window vs. patch-and-deprecate).
• A forward-looking note on automating publish via a tag-triggered workflow with NPM_TOKEN, if/when there's appetite.

Referenced from AGENTS.md §9.

Things that are deliberately not in this PR

• Auto-publishing on tag push. Mentioned at the bottom of RELEASING.md as a future improvement — would require an NPM_TOKEN secret and ownership conversation. Easy follow-up once we're sure the manual flow is right.
• Dependabot config update. The repo's existing dependabot is producing PRs against the old Travis-era setup. After this merges, dependabot will start opening PRs for npm only (which is what we want anyway). No .github/dependabot.yml is being added — relying on GitHub's defaults.
• Branch protection rules. Those live in repo settings, not in code; not something this PR can change.
• Promoting ember-release back to a hard requirement. Will happen as part of the Ember 5/6 migration work.