v0.3.0

Added

• cairnlock verify --base <git-ref>: reads cairnlock.lock as it exists at
  the given ref with git show and compares the working tree against it. A
  pull request that edits a tracked file and rehashes the lockfile in the
  same change now fails with "lockfile modified in this change".
• --allow-lock-change flag to permit lockfile-only changes.
• Exit code 2 for usage and environment errors in verify: refs starting with
  - are rejected before git runs, missing base lockfiles suggest running
  cairnlock init on the base branch, and shallow clones get a
  fetch-depth: 0 hint for GitHub Actions.
• All git calls use an argument list, never a shell.
• base-ref and allow-lock-change inputs in action.yml, with a verify
  step that skips cleanly when no lockfile exists yet.
• Marketplace metadata in action.yml (name, description, branding).
• Documentation site in docs/: landing page, guide (lockfile workflow,
  base verification, CODEOWNERS, CI), and a full rule reference. Plain HTML,
  no build step.
• docs/rules.md as the SARIF helpUri target for every rule id.
• SECURITY.md with reporting instructions and scope.
• Dependabot config for pip and GitHub Actions.
• README section recommending CODEOWNERS protection for cairnlock.lock.

Changed

• verify without a lockfile now exits 2 (usage/environment error) instead of 1.
• CLI status markers are plain text (OK:, error:, warning:) instead of
  emoji, per the project style rules.
• SARIF driver version now comes from the package version instead of a
  hard-coded string.
• Ruff ignores E501; long regex pattern tables and embedded report templates
  stay readable without manual line splitting.

Fixed

• CI lint failures: ruff check passed locally on every configuration
  tested before this release.