fix: split perf report workflow for fork PR support (#9382)

## Summary

Perf report workflow fails on fork PRs because `GITHUB_TOKEN` is
read-only for forks, causing "Resource not accessible by integration" on
the PR comment step.

## Changes

- **What**: Split `ci-perf-report.yaml` into a data-collection workflow
+ a `workflow_run`-triggered reporter (`pr-perf-report.yaml`), matching
the existing `ci-size-data`/`pr-size-report` pattern. Added fork PR
permissions guidance to `.github/AGENTS.md`.
- **ci-perf-report.yaml**: Removed the `report` job and `pull-requests:
write` permission. Added PR metadata (number + base branch) artifact
upload.
- **pr-perf-report.yaml** (new): Triggered by `workflow_run` on the perf
workflow. Downloads metrics + metadata artifacts, generates report,
posts PR comment with write permissions from the default-branch context.

## Review Focus

- The two-workflow split follows the same pattern as `ci-size-data.yaml`
→ `pr-size-report.yaml`, which already works for fork PRs.
- The `workflow_run` trigger runs in the base repo context per [GitHub
Security Lab
guidance](https://securitylab.github.com/resources/github-actions-preventing-pwn-requests/),
so it safely has write permissions even for fork PRs.
- AGENTS.md guidance documents this pattern to prevent recurrence.

Fixes the failure seen in
https://github.com/Comfy-Org/ComfyUI_frontend/actions/runs/22684230751/job/65763595989?pr=9380

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9382-fix-split-perf-report-workflow-for-fork-PR-support-3196d73d365081b29b35ed354e7789e2)
by [Unito](https://www.unito.io)

.github/actions/post-pr-report-comment/action.yaml

@@ -0,0 +1,35 @@
+name: Post PR Report Comment
+description: Reads a markdown report file and posts/updates a single idempotent comment on a PR.
+
+inputs:
+  pr-number:
+    description: PR number to comment on
+    required: true
+  report-file:
+    description: Path to the markdown report file
+    required: true
+  comment-marker:
+    description: HTML comment marker for idempotent matching
+    required: true
+  token:
+    description: GitHub token with pull-requests write permission
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Read report
+      id: report
+      uses: juliangruber/read-file-action@b549046febe0fe86f8cb4f93c24e284433f9ab58 # v1.1.7
+      with:
+        path: ${{ inputs.report-file }}
+
+    - name: Create or update PR comment
+      uses: actions-cool/maintain-one-comment@4b2dbf086015f892dcb5e8c1106f5fccd6c1476b # v3.2.0
+      with:
+        token: ${{ inputs.token }}
+        number: ${{ inputs.pr-number }}
+        body: |
+          ${{ steps.report.outputs.content }}
+          ${{ inputs.comment-marker }}
+        body-include: ${{ inputs.comment-marker }}