fix: correct broken links and image references in documentation

- Remove broken link to non-existent how-to-customize page in plan mode docs
- Fix incorrect image reference for plan mode selector
- Correct assistant extension selector image path to match actual file location

Author: bdougie

docs/features/plan/how-it-works.mdx

@@ -85,8 +85,7 @@ Plan mode uses a dedicated system message that:
 - Encourages thorough analysis before suggesting changes
 
 <Info>
-  The system message can be customized per model. See [How to
-  Customize](/features/plan/how-to-customize) for details or see it [directly in
+  The system message can be customized per model. See it [directly in
   the
   code](https://github.com/continuedev/continue/blob/main/core/llm/defaultSystemMessages.ts).
 </Info>

docs/features/plan/quick-start.mdx

@@ -11,7 +11,7 @@ Plan mode provides a safe environment for understanding and constructing plans w
 
 You can switch to `Plan` in the mode selector below the chat input box.
 
-![How to select plan mode](/images/mode-select-plan.png)
+![How to select plan mode](/images/plan-mode-selector.png)
 
 <Info>
   If Plan is disabled with a `Not Supported` message, the selected model or

docs/hub/assistants/use-an-assistant.mdx

@@ -16,4 +16,4 @@ After saving, open your IDE extension and:
 - Select the assistant from the **assistant dropdown** in the Continue extension.
 - Begin using it for chat, code generation, or other configured capabilities.
 
-[Extension Assistant Selector](/images/assistant-extension-select-319492a06d6249e5389747687341dfdb.png)
+![Extension Assistant Selector](/images/hub/assistants/images/assistant-extension-select.png)