feat: reusable ci.yml — estate CI standard (Phase 0, step 1)

Add .github/workflows/ci.yml as a reusable workflow_call entrypoint for
all 7 estate repos. Consistent job names (lint-php, phpcs, file-size,
phpunit, lint-js) so deploy-app.yml can needs: them uniformly.

Inputs: php_versions (JSON matrix), tests (stubs|wp-framework|brain-monkey|none),
enforce_300, line_limit_paths, has_js, phpcs_autofix.

phpcbf auto-commit: on pull_request, phpcbf runs and commits any auto-fixable
PHPCS violations back to the branch via GITHUB_TOKEN before strict phpcs.
On push events the commit step is skipped to prevent loops.

wp-framework PHPUnit path handles all three known WP CI gotchas:
(1) install svn (ubuntu-latest lacks it);
(2) skip-db-create=true (MySQL service pre-creates wp_tests via MYSQL_DATABASE);
(3) WP_CORE_DIR trailing slash (ABSPATH concatenation guard).

Also adds:
- .github/workflows/ci-self-test.yml: self-test caller (tests: none) so the
  reusable workflow is exercised on this PR.
- README.md: adoption snippet + full input reference for caller repos.
- CHANGELOG.md: v1.6.0 entry.

Agent-Session: claude/reusable-ci-20260616

.github/workflows/ci-self-test.yml

@@ -0,0 +1,23 @@
+# Self-test: exercises the reusable ci.yml on this repo itself.
+# Uses tests: none (peptide-e2e is a Playwright/Node repo, no PHP suite)
+# and has_js: false (the JS is TypeScript, not linted via npm run lint here).
+# Purpose: smoke-check that the reusable workflow parses and runs without
+# errors on every push/PR to this repo.
+
+name: CI Self-Test (reusable ci.yml)
+
+on:
+  push:
+    branches: [main, 'claude/**']
+  pull_request:
+    branches: [main]
+
+jobs:
+  ci:
+    uses: peptiderepo/peptide-e2e/.github/workflows/ci.yml@claude/reusable-ci-20260616
+    with:
+      tests: none
+      enforce_300: false
+      has_js: false
+      phpcs_autofix: false
+    secrets: inherit