ci(release): release on every merge to main with semantic-release

[onamfc]

Description

Every merge to main now releases when its commits call for one, the same way LinkForty Core does.

• .github/workflows/release.yml runs on push to main instead of on a tag. It lints, runs phpcs, then npx semantic-release.
• .releaserc.json uses Core's rules: feat → minor; fix / docs / refactor / perf / revert → patch; chore / test / ci / build → none. One difference: a breaking change bumps the minor version while the plugin is below 1.0 (semver allows this for 0.x). At 1.0, change that rule to major.
• On a release:
  1. scripts/set-version.sh writes the version into the plugin header, the LINKFORTY_VERSION constant and readme.txt's Stable tag. It exits non-zero if any of the three doesn't match.
  2. @semantic-release/changelog prepends the release notes to CHANGELOG.md.
  3. composer build makes linkforty-x.y.z.zip.
  4. @semantic-release/git commits chore(release): x.y.z [skip ci].
  5. @semantic-release/github tags vx.y.z and publishes the release with the zip attached.
• package.json is private and holds the release tooling only. It, package-lock.json, .releaserc.json and scripts/ are added to .distignore, so the zip is unchanged (checked with composer build).
• CONTRIBUTING.md and the PR template now say:
  • PRs are squash-merged, so the PR title becomes the release commit and its changelog line.
  • A breaking change needs ! and a BREAKING CHANGE: note.
  • Nobody edits CHANGELOG.md or the version by hand.

Catching up 0.2.0

PR #1 (purchase revenue, begin_checkout → cart_total) was merged without a release, and its commit isn't conventional, so semantic-release wouldn't release it. This PR sets 0.2.0 by hand: the header, the constant, Stable tag, and ## [0.2.0] in the changelog. After merging, tag the merge commit v0.2.0 and publish that release once by hand. semantic-release then counts from v0.2.0.

Use a squash merge so the title above is the commit on main. It's a ci: commit, so the release job runs and correctly releases nothing.

Type of change

• Refactor or tooling

Testing

• scripts/set-version.sh 9.8.7 writes all three places; scripts/set-version.sh nope exits 1
• npx semantic-release --dry-run on this branch finds v0.1.0, analyzes the 4 commits since it, and reports no release (none are feat/fix), as expected
• composer build zip contains only the plugin files
• composer lint and composer phpcs pass; the CI header check passes at 0.2.0

Checklist

• The title is a conventional commit
• README and readme.txt unchanged (no setting, event or hook changed)