feat(semconv): add conventional pull request action

.config/mise/tasks/github/actions/ci/lint

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+#!/usr/bin/env bash
+#MISE description="Lint every workflow and shell script"
+set -euo pipefail
+
+actionlint
+
+# -r keeps this a no-op rather than an error when nothing matches.
+git ls-files -z '*.sh' '.config/mise/tasks/**' | xargs -0 -r shellcheck -x

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions: {}
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  check:
+    name: Lint
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      # Tool versions live in mise.toml and the task lives under
+      # .config/mise/tasks, so a contributor runs exactly what CI runs.
+      - uses: jdx/mise-action@c2a87611a18de5b3828c5652fe268e992400cb5c # v4.3.0
+
+      - run: mise run github:actions:ci:lint

.github/workflows/semconv.yml

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+name: SemConv
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+permissions: {}
+
+jobs:
+  lint-pr-title:
+    name: Validate PR Title
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    permissions:
+      contents: read
+      pull-requests: read
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - uses: ./actions/semconv/pull-request

actions/semconv/README.md

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+# semconv
+
+Actions that enforce [Conventional Commits][spec] across a repository.
+
+| Action | Enforces |
+| --- | --- |
+| [`pull-request`](pull-request) | The pull request title |
+
+[spec]: https://www.conventionalcommits.org/en/v1.0.0/

actions/semconv/pull-request/README.md

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+# semconv/pull-request
+
+Fails a pull request whose title is not a Conventional Commit. A thin wrapper
+around [`amannn/action-semantic-pull-request`][upstream] that fixes the grammar
+so every repository agrees on it.
+
+## Usage
+
+The job name matters. Repository rulesets require the check by the literal
+string `Validate PR Title`, so renaming the job silently stops the required
+check from ever reporting.
+
+```yaml
+name: SemConv
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+permissions: {}
+
+jobs:
+  lint-pr-title:
+    name: Validate PR Title
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    permissions:
+      pull-requests: read
+    steps:
+      - uses: TrogonStack/github-actions/actions/semconv/pull-request@<full-sha>
+```
+
+The reference must be a full commit SHA. Both organizations set
+`sha_pinning_required`, so a branch or tag reference makes the job refuse to
+start rather than fail.
+
+## Grammar
+
+```
+<type>[(scope)][!]: <description>
+```
+
+`feat`, `fix`, and `chore` are the only types. Anything that is not a feature or
+a fix is a chore. The description must not start with an uppercase letter.
+
+The type list is not an input. One grammar for every repository is the reason
+this action exists; a knob invites back the divergence it was built to remove.
+
+## Inputs
+
+| Input | Default | Description |
+| --- | --- | --- |
+| `extra-ignore-labels` | none | Further labels that skip the check, one per line. |
+| `token` | `github.token` | Read access is sufficient. |
+
+`bot`, `dependencies`, and `autorelease: pending` are always ignored. The last
+is release-please's own pull request, which does not follow the grammar it
+exists to produce.
+
+## Title only
+
+Only the title is checked, because only the title reaches the default branch on
+a squash merge. Repositories that allow merge or rebase commits, or that squash
+with `COMMIT_OR_PR_TITLE`, can still land an unconventional subject. That is a
+repository settings problem and is fixed in the Terraform that owns those
+settings, not here.
+
+[upstream]: https://github.com/amannn/action-semantic-pull-request

actions/semconv/pull-request/action.yml

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+name: Validate PR Title
+description: >-
+  Fails a pull request whose title is not a Conventional Commit.
+author: TrogonStack
+
+inputs:
+  extra-ignore-labels:
+    description: >-
+      Further labels that skip the check, one per line. Bot and release labels
+      are already ignored.
+    required: false
+    default: ''
+  token:
+    description: Token used to read the pull request. Read access is sufficient.
+    required: false
+    default: ${{ github.token }}
+
+runs:
+  using: composite
+  steps:
+    - name: Validate PR title
+      uses: amannn/action-semantic-pull-request@48f256284bd46cdaab1048c3721360e808335d50 # v6.1.1
+      env:
+        GITHUB_TOKEN: ${{ inputs.token }}
+      with:
+        # Anything that is not a feature or a fix is a chore. This list is
+        # deliberately not an input: one grammar for every repository is the
+        # reason this action exists.
+        types: |
+          feat
+          fix
+          chore
+        requireScope: false
+        subjectPattern: ^(?![A-Z]).+$
+        subjectPatternError: |
+          The subject "{subject}" in the pull request title "{title}" should start
+          with a lowercase letter.
+        # "autorelease: pending" is release-please's own pull request, which does
+        # not follow the grammar it exists to produce.
+        ignoreLabels: |
+          bot
+          dependencies
+          autorelease: pending
+          ${{ inputs.extra-ignore-labels }}

mise.toml

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+[tools]
+actionlint = "1.7.12"
+shellcheck = "0.11.0"