fix: stop GitHub from evaluating example expressions in action.yml

GitHub's runner evaluates `${{ ... }}` tokens that appear anywhere in
`action.yml` — including inside `description:` blocks — when it loads
the action manifest. Embedding `${{ github.sha }}` and
`${{ secrets.OPENAI_API_KEY }}` literally as documentation examples
caused every workflow that referenced this action to fail at load time
with `Unrecognized named-value: 'github'` and `'secrets'`.

Replace the four offending example tokens with placeholders that don't
resemble Actions expressions:

- `tags` example: `commit: ${{ github.sha }}` → `commit: <commit-sha>`
- `branch` description: ``Defaults to `${{ github.ref_name }}`.`` →
  ``Defaults to the `GITHUB_REF_NAME` environment variable.``
- `commit-sha` description: same treatment with `GITHUB_SHA`.
- `secrets` example: `KEY=${{ secrets.* }}` → `KEY=<placeholder>`.

Real workflow examples in README.md and examples/ are unaffected
because expressions inside workflow files are evaluated normally.

Author: lukekim

action.yml

@@ -47,7 +47,7 @@ inputs:
         tags: |
           environment: production
           team: data-platform
-          commit: ${{ github.sha }}
+          commit: <commit-sha>
 
       JSON form:
         tags: '{"environment":"production","team":"data-platform"}'
@@ -74,10 +74,10 @@ inputs:
     description: Number of replicas for this deployment (1-10).
     required: false
   branch:
-    description: Git branch attributed to the deployment. Defaults to `${{ github.ref_name }}`.
+    description: Git branch attributed to the deployment. Defaults to the `GITHUB_REF_NAME` environment variable.
     required: false
   commit-sha:
-    description: Git commit SHA attributed to the deployment. Defaults to `${{ github.sha }}`.
+    description: Git commit SHA attributed to the deployment. Defaults to the `GITHUB_SHA` environment variable.
     required: false
   commit-message:
     description: Commit message for the deployment. Defaults to the head-commit message of the GitHub event.
@@ -93,8 +93,8 @@ inputs:
 
       Example:
         secrets: |
-          OPENAI_API_KEY=${{ secrets.OPENAI_API_KEY }}
-          PG_PASSWORD=${{ secrets.PG_PASSWORD }}
+          OPENAI_API_KEY=<openai-api-key>
+          PG_PASSWORD=<pg-password>
     required: false
   wait-for-completion:
     description: Poll the deployment until it succeeds or fails.