ci(docs-claude-review): Update comments instead of creating new ones (#12106)

## Changes 🏗️

This PR updates the Claude Block Docs Review CI workflow to update
existing comments instead of creating new ones on each push.

### What's Changed:
1. **Concurrency group** - Prevents race conditions if the workflow runs
twice simultaneously
2. **Comment cleanup step** - Deletes any previous Claude review comment
before posting a new one
3. **Marker instruction** - Instructs Claude to include a `<!--
CLAUDE_DOCS_REVIEW -->` marker in its comment for identification

### Why:
Previously, every PR push would create a new review comment, cluttering
the PR with multiple comments. Now only the most recent review is shown.

### Testing:
1. Create a PR that triggers this workflow (modify a file in
`docs/integrations/` or `autogpt_platform/backend/backend/blocks/`)
2. Verify first run creates comment with marker
3. Push another commit
4. Verify old comment is deleted and new comment is created (not
accumulated)

Requested by @Bentlybro

---

## Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [ ] I have made a test plan
- [ ] I have tested my changes according to the test plan (will be
tested on merge)

#### For configuration changes:
- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)

<!-- greptile_comment -->

<h2>Greptile Overview</h2>

<details><summary><h3>Greptile Summary</h3></summary>

Added concurrency control and comment deduplication to prevent multiple
Claude review comments from accumulating on PRs. The workflow now
deletes previous review comments (identified by `<!-- CLAUDE_DOCS_REVIEW
-->` marker) before posting new ones, and uses concurrency groups to
prevent race conditions.
</details>


<details><summary><h3>Confidence Score: 5/5</h3></summary>

- This PR is safe to merge with minimal risk
- The changes are well-contained, follow GitHub Actions best practices,
and use built-in GitHub APIs safely. The concurrency control prevents
race conditions, and the comment cleanup logic uses proper filtering
with `head -1` to handle edge cases. The HTML comment marker approach is
standard and reliable.
- No files require special attention
</details>


<details><summary><h3>Sequence Diagram</h3></summary>

```mermaid
sequenceDiagram
    participant GH as GitHub PR Event
    participant WF as Workflow
    participant API as GitHub API
    participant Claude as Claude Action
    
    GH->>WF: PR opened/synchronized
    WF->>WF: Check concurrency group
    Note over WF: Cancel any in-progress runs<br/>for same PR number
    WF->>API: Query PR comments
    API-->>WF: Return all comments
    WF->>WF: Filter for CLAUDE_DOCS_REVIEW marker
    alt Previous comment exists
        WF->>API: DELETE comment by ID
        API-->>WF: Comment deleted
    else No previous comment
        WF->>WF: Skip deletion
    end
    WF->>Claude: Run code review
    Claude->>API: POST new comment with marker
    API-->>Claude: Comment created
```
</details>


<sub>Last reviewed commit: fb1b436</sub>

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