fix(backend): Rename LINEAR_API_KEY to COPILOT_LINEAR_API_KEY to prevent global access (#12143)

The `LINEAR_API_KEY` environment variable name is too generic — it
matches the key name used by integrations/blocks, meaning that if set
globally, it could inadvertently grant all users access to Linear
through the blocks system rather than restricting it to the copilot
feature-request tool.

This renames the setting to `COPILOT_LINEAR_API_KEY` to make it clear
this key is scoped exclusively to the copilot's feature-request
functionality, preventing it from being picked up as a general-purpose
Linear credential.

### Changes 🏗️

- Renamed `linear_api_key` → `copilot_linear_api_key` in `Secrets`
settings model (`backend/util/settings.py`)
- Updated all references in the copilot feature-request tool
(`backend/api/features/chat/tools/feature_requests.py`)

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] Verified the rename is consistent across all references (settings
+ feature_requests tool)
  - [x] No other files reference the old `linear_api_key` setting name

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

> **Note:** The env var changes from `LINEAR_API_KEY` to
`COPILOT_LINEAR_API_KEY`. Any deployment using the old name will need to
update accordingly.

<!-- greptile_comment -->

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

Renamed `LINEAR_API_KEY` to `COPILOT_LINEAR_API_KEY` in settings and the
copilot feature-request tool to prevent unintended access through Linear
blocks.

**Key changes:**
- Updated `Secrets.linear_api_key` → `Secrets.copilot_linear_api_key` in
`backend/util/settings.py`
- Updated all references in
`backend/api/features/chat/tools/feature_requests.py`
- The rename prevents the copilot Linear key from being picked up by the
Linear blocks integration (which uses `LINEAR_API_KEY` via
`ProviderBuilder` in `backend/blocks/linear/_config.py`)

**Issues found:**
- `.env.default` still references `LINEAR_API_KEY` instead of
`COPILOT_LINEAR_API_KEY`
- Frontend styleguide has a hardcoded error message with the old
variable name
</details>


<details><summary><h3>Confidence Score: 3/5</h3></summary>

- Generally safe but requires fixing `.env.default` before deployment
- The code changes are correct and achieve the intended security
improvement by preventing scope leakage. However, the PR is incomplete -
`.env.default` wasn't updated (critical for deployment) and a frontend
error message reference was missed. These issues will cause
configuration problems for anyone deploying with the new variable name.
- Check `autogpt_platform/backend/.env.default` and
`autogpt_platform/frontend/src/app/(platform)/copilot/styleguide/page.tsx`
- both need updates to match the renamed variable
</details>


<details><summary><h3>Flowchart</h3></summary>

```mermaid
flowchart TD
    A[".env file<br/>COPILOT_LINEAR_API_KEY"] --> B["Secrets model<br/>copilot_linear_api_key"]
    B --> C["feature_requests.py<br/>_get_linear_config()"]
    C --> D["Creates APIKeyCredentials<br/>for copilot feature requests"]
    
    E[".env file<br/>LINEAR_API_KEY"] --> F["ProviderBuilder<br/>in blocks/linear/_config.py"]
    F --> G["Linear blocks integration<br/>for user workflows"]
    
    style A fill:#90EE90
    style B fill:#90EE90
    style C fill:#90EE90
    style D fill:#90EE90
    style E fill:#FFD700
    style F fill:#FFD700
    style G fill:#FFD700
```
</details>


<sub>Last reviewed commit: 86dc57a</sub>

<!-- greptile_other_comments_section -->

<!-- /greptile_comment -->

autogpt_platform/backend/backend/api/features/chat/tools/feature_requests.py

@@ -104,8 +104,8 @@ def _get_linear_config() -> tuple[LinearClient, str, str]:
     Raises RuntimeError if any required setting is missing.
     """
     secrets = _get_settings().secrets
-    if not secrets.linear_api_key:
-        raise RuntimeError("LINEAR_API_KEY is not configured")
+    if not secrets.copilot_linear_api_key:
+        raise RuntimeError("COPILOT_LINEAR_API_KEY is not configured")
     if not secrets.linear_feature_request_project_id:
         raise RuntimeError("LINEAR_FEATURE_REQUEST_PROJECT_ID is not configured")
     if not secrets.linear_feature_request_team_id:
@@ -114,7 +114,7 @@ def _get_linear_config() -> tuple[LinearClient, str, str]:
     credentials = APIKeyCredentials(
         id="system-linear",
         provider="linear",
-        api_key=SecretStr(secrets.linear_api_key),
+        api_key=SecretStr(secrets.copilot_linear_api_key),
         title="System Linear API Key",
     )
     client = LinearClient(credentials=credentials)

autogpt_platform/backend/backend/util/settings.py

@@ -662,7 +662,7 @@ class Secrets(UpdateTrackingModel["Secrets"], BaseSettings):
     mem0_api_key: str = Field(default="", description="Mem0 API key")
     elevenlabs_api_key: str = Field(default="", description="ElevenLabs API key")
 
-    linear_api_key: str = Field(
+    copilot_linear_api_key: str = Field(
         default="", description="Linear API key for system-level operations"
     )
     linear_feature_request_project_id: str = Field(