Docs: document per-project language_backend override; move field to SharedConfig

- Add "Per-Project Configuration" section to JetBrains plugin docs
  with cross-reference anchor for linking from other pages
- Add language_backend to project configuration feature list in workflow docs
- Add cross-reference from global config docs to per-project override
- Move language_backend field from ProjectConfig to SharedConfig,
  following the same pattern as symbol_info_budget (review feedback)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/02-usage/025_jetbrains_plugin.md

@@ -48,10 +48,31 @@ language_backend: JetBrains
 *Note*: you can also use the button `Edit Global Serena Config` in the Serena MCP dashboard to open the config file in your default editor.
 
 **Per-Instance Configuration**.
-The configuration setting in the global config file can be overridden on a 
-per-instance basis by providing the arguments `--language-backend JetBrains` when 
+The configuration setting in the global config file can be overridden on a
+per-instance basis by providing the arguments `--language-backend JetBrains` when
 launching the Serena MCP server.
 
+(per-project-language-backend)=
+**Per-Project Configuration**.
+You can also set the language backend on a per-project basis in the project's
+`.serena/project.yml` file:
+
+```yaml
+language_backend: JetBrains
+```
+
+If set, this overrides the global `language_backend` setting for the session when the project is
+activated at startup (via the `--project` flag).
+
+:::{important}
+The language backend is determined once at startup and cannot be changed during a running session.
+If a project with a different backend is activated after startup, Serena will return an error.
+
+If you need to work with projects that use different backends, you can either:
+1. Use the `--project` flag to activate the project at startup, which will use its configured backend.
+2. Configure separate MCP server instances (one per backend) in your client.
+:::
+
 **Verifying the Setup**.
 You can verify that Serena is using the JetBrains plugin by either checking the dashboard, where
 you will see `Languages:

docs/02-usage/040_workflow.md

@@ -46,12 +46,13 @@ After creation, you can adjust the project settings in the generated `.serena/pr
 within the project directory.
 
 The file allows you to configure ...
-  * the set of programming languages for which language servers are spawned (not relevant when using the JetBrains plugin)  
+  * the set of programming languages for which language servers are spawned (not relevant when using the JetBrains plugin)
     Note that you can dynamically add/remove language servers while Serena is running via the [Dashboard](060_dashboard).
+  * the [language backend](per-project-language-backend) to use for this project (overriding the global setting)
   * the encoding used in source files
   * ignore rules
   * write access
-  * an initial prompt that shall be passed to the LLM whenever the project is activated 
+  * an initial prompt that shall be passed to the LLM whenever the project is activated
   * the name by which you want to refer to the project (relevant when telling the LLM to dynamically activate the project)
   * the set of tools and modes to use by default
 

docs/02-usage/050_configuration.md

@@ -23,7 +23,8 @@ The global configuration file allows you to change general settings and defaults
 ### Settings
 
 Some of the configurable settings include:
-  * the language backend to use by default (i.e., the JetBrains plugin or language servers)
+  * the language backend to use by default (i.e., the JetBrains plugin or language servers);
+    this can also be [overridden per project](per-project-language-backend)
   * UI settings affecting the [Serena Dashboard and GUI tool](060_dashboard.md)
   * the set of tools to enable/disable by default
   * the set of modes to use by default