docs(cicd): pin reusable-workflow refs to commit SHAs instead of @main

The org standard for reusable-workflow references changes from the mutable
@main branch ref to full commit SHA pins advanced by Dependabot. Adds a
Workflow Ref Pinning section covering the rationale and the two
prerequisites that keep pins current (github-actions ecosystem in
dependabot.yml, org-level Dependabot access to the internal .github repo).

cicd.md

@@ -27,7 +27,7 @@ on:
     branches: [main]
 jobs:
   ci:
-    uses: Sea-Haven-Industries/.github/.github/workflows/ci-typescript-cdk.yaml@main
+    uses: Sea-Haven-Industries/.github/.github/workflows/ci-typescript-cdk.yaml@<full-commit-sha>  # main
     with:
       node-version: "24"
 
@@ -38,7 +38,7 @@ on:
     branches: [main]
 jobs:
   deploy:
-    uses: Sea-Haven-Industries/.github/.github/workflows/cd-cdk.yaml@main
+    uses: Sea-Haven-Industries/.github/.github/workflows/cd-cdk.yaml@<full-commit-sha>  # main
     with:
       node-version: "24"
     secrets:
@@ -55,7 +55,7 @@ on:
     branches: [main]
 jobs:
   ci:
-    uses: Sea-Haven-Industries/.github/.github/workflows/ci-python-sam.yaml@main
+    uses: Sea-Haven-Industries/.github/.github/workflows/ci-python-sam.yaml@<full-commit-sha>  # main
 
 # .github/workflows/deploy.yaml
 name: Deploy
@@ -64,7 +64,7 @@ on:
     branches: [main]
 jobs:
   deploy:
-    uses: Sea-Haven-Industries/.github/.github/workflows/cd-sam.yaml@main
+    uses: Sea-Haven-Industries/.github/.github/workflows/cd-sam.yaml@<full-commit-sha>  # main
     with:
       stack-name: "your-stack-name"
     secrets:
@@ -88,7 +88,24 @@ Always pass `node-version: "24"` to reusable workflows. Local dev uses Node 24 /
 ## Naming
 
 - All workflow files: kebab-case
-- Reusable workflow references: `@main` branch
+- Reusable workflow references: pinned to a full 40-character commit SHA with a `# main` comment — never a branch or tag ref
+
+## Workflow Ref Pinning
+
+Reusable workflow references are pinned to a full commit SHA of the central `.github` repo, with a trailing `# main` comment:
+
+```yaml
+uses: Sea-Haven-Industries/.github/.github/workflows/ci-python-sam.yaml@<full-commit-sha>  # main
+```
+
+Branch refs are mutable: a compromised or bad commit on the central repo would flow instantly into every consumer's CI and deploy path. A SHA pin turns that same change into a reviewable Dependabot PR instead. The comment tells Dependabot (and readers) which ref the pin tracks.
+
+Per the pinning principle, pins are for reproducibility, not for freezing time. Two prerequisites keep them moving:
+
+1. Every repo's `dependabot.yml` must include the `github-actions` ecosystem (weekly), so pin-advance PRs are opened automatically.
+2. Dependabot must be granted access to the internal `.github` repo at the org level (Org Settings → Advanced Security → Global settings → "Grant Dependabot access to repositories"). Without the grant, update jobs fail with `git_dependencies_not_reachable` and pins freeze silently — consumers stop receiving central workflow fixes with no visible signal beyond the failed Dependabot run.
+
+When adding a caller workflow by hand, pin to the current tip of the central repo's `main` (`gh api /repos/Sea-Haven-Industries/.github/commits/main --jq .sha`) and let Dependabot advance it from there.
 
 ## When to Add a Pipeline
 
@@ -113,7 +130,7 @@ permissions:
   issues: write
 jobs:
   label:
-    uses: Sea-Haven-Industries/.github/.github/workflows/callable-labeler.yaml@main
+    uses: Sea-Haven-Industries/.github/.github/workflows/callable-labeler.yaml@<full-commit-sha>  # main
 ```
 
 - The trigger is plain `pull_request`, not `pull_request_target`: private repos take no fork PRs, so the lower-privilege event is sufficient and avoids the pwn-request surface. Because `pull_request` runs the workflow from the merge commit, the Labeler check appears on the PR that first adds the caller — an absent or failed check means a missing permission, not expected behaviour.