ci: adds a reusable workflow for linking issues

.github/docs/Link Issues by Milestone.md

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+# Link Issue to Cross-Repo Milestone Parent
+
+**Name:** `Link issue to cross-repo milestone parent`
+
+**Workflow File:** `util-link-issues-by-milestone.yml`
+
+**Description:**
+
+This workflow links a milestoned issue as a sub-issue of a parent issue in a target repository. It is designed to be triggered via `workflow_call` from a per-repo workflow that listens for the `issues: [milestoned]` event. The parent issue is identified by parsing the milestone's description, which is expected to contain a URL ending in `/issues/<number>`.
+
+**Created By:** AIND Scientific Computing
+
+## Parameters
+
+**Inputs:**
+
+- `issue-number` (required): Issue number to link as a sub-issue
+- `issue-id` (required): Issue node ID to link as a sub-issue
+- `milestone-description` (required): Milestone description containing the parent issue URL
+- `target-owner` (optional): Owner of the repository containing the parent issue
+  - default: `AllenNeuralDynamics`
+- `target-repo` (optional): Repository name containing the parent issue
+  - default: `aind-scientific-computing`
+
+**Secrets:**
+
+- `service-token` (required): Token with permission to read and link issues across repositories
+
+**Outputs:** N/A
+
+## Example
+
+**workflow.yml**
+```yml
+name: Link Issue to Milestone Parent
+
+on:
+  issues:
+    types: [milestoned]
+
+jobs:
+  link-issue:
+    if: github.event.issue.milestone != null
+    uses: AllenNeuralDynamics/.github/.github/workflows/util-link-issues-by-milestone.yml@main
+    with:
+      issue-number: ${{ github.event.issue.number }}
+      issue-id: ${{ github.event.issue.id }}
+      milestone-description: ${{ github.event.issue.milestone.description }}
+    secrets:
+      service-token: ${{ secrets.SERVICE_TOKEN }}

.github/workflows/util-link-issues-by-milestone.yml

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+name: Link issue to cross-repo milestone parent
+
+on:
+  workflow_call:
+    inputs:
+      issue-number:
+        description: 'Issue number to link as a sub-issue'
+        required: true
+        type: number
+      issue-id:
+        description: 'Issue ID to link as a sub-issue'
+        required: true
+        type: number
+      milestone-description:
+        description: 'Milestone description containing the parent issue URL'
+        required: true
+        type: string
+      target-owner:
+        description: 'Owner of the repository containing the parent issue'
+        required: false
+        type: string
+        default: 'AllenNeuralDynamics'
+      target-repo:
+        description: 'Repository name containing the parent issue'
+        required: false
+        type: string
+        default: 'aind-scientific-computing'
+    secrets:
+      service-token:
+        required: true
+
+jobs:
+  link:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Link to parent issue
+        uses: actions/github-script@v7
+        env:
+          ISSUE_NUMBER: ${{ inputs.issue-number }}
+          ISSUE_ID: ${{ inputs.issue-id }}
+          TARGET_OWNER: ${{ inputs.target-owner }}
+          TARGET_REPO: ${{ inputs.target-repo }}
+          MILESTONE_DESC: ${{ inputs.milestone-description }}
+        with:
+          github-token: ${{ secrets.service-token }}
+          script: |
+            const issueNumber = parseInt(process.env.ISSUE_NUMBER);
+            const issueId = parseInt(process.env.ISSUE_ID);
+            const targetOwner = process.env.TARGET_OWNER;
+            const targetRepo = process.env.TARGET_REPO;
+            const url = process.env.MILESTONE_DESC;
+
+            const match = url?.match(/\/issues\/(\d+)$/);
+
+            if (!match) {
+              console.log(`Milestone description is not a roadmap URL: ${url}`);
+              return;
+            }
+
+            const parentNumber = parseInt(match[1]);
+
+            const { data: parent } = await github.rest.issues.get({
+              owner: targetOwner,
+              repo: targetRepo,
+              issue_number: parentNumber
+            });
+
+            if (!parent) {
+              console.log(`No issue found at ${url}`);
+              return;
+            }
+
+            await github.request("POST /repos/{owner}/{repo}/issues/{issue_number}/sub_issues", {
+              owner: targetOwner,
+              repo: targetRepo,
+              issue_number: parentNumber,
+              sub_issue_id: issueId,
+              headers: {
+                "X-GitHub-Api-Version": "2022-11-28"
+              }
+            });
+
+            console.log(`Linked issue #${issueNumber} as sub-issue of ${targetRepo}#${parentNumber}`);