ci: add CI workflow with fmt, validate, docs, lint, and release-please jobs

Replace the standalone release-please workflow with a unified CI
workflow that runs formatting checks, terraform validate, docs
freshness, and tflint on every push. Release-please is now handled
by jimeh/release-please-manifest-action with GitHub App auth.

Also:
- Move release-please config files under .github/
- Add changelog-sections and always-update to release-please config
- Add tflint with recommended terraform plugin preset
- Add .gitignore for .terraform/ and .terraform.lock.hcl
- Add lint and lint-fix Make targets
- Clean .terraform.lock.hcl before terraform-docs runs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,49 @@
+---
+name: CI
+on: [push]
+
+permissions:
+  contents: read
+
+jobs:
+  fmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: jdx/mise-action@6d1e696aa24c1aa1bcc1adea0212707c71ab78a8 # v3.6.1
+      - run: terraform fmt -check -recursive -diff
+      - run: prettier --check README.md
+
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: jdx/mise-action@6d1e696aa24c1aa1bcc1adea0212707c71ab78a8 # v3.6.1
+      - run: terraform init -backend=false
+      - run: terraform validate
+
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: jdx/mise-action@6d1e696aa24c1aa1bcc1adea0212707c71ab78a8 # v3.6.1
+      - run: terraform-docs --output-check .
+      - run: prettier --check README.md
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: jdx/mise-action@6d1e696aa24c1aa1bcc1adea0212707c71ab78a8 # v3.6.1
+      - run: tflint --init
+      - run: tflint --format compact
+
+  release-please:
+    runs-on: ubuntu-latest
+    if: startsWith(github.ref, 'refs/heads/')
+    steps:
+      - uses: jimeh/release-please-manifest-action@84f33fd2828210488c36f3e0a7e3209252d2ae7d # v3.0.0
+        with:
+          target-branch-pattern: "^(main|master)$"
+          app-id: ${{ secrets.RELEASE_BOT_APP_ID }}
+          private-key: ${{ secrets.RELEASE_BOT_PRIVATE_KEY }}