Simplify CI config (#4117)

Some cleanup in preparation for
#4118

Set default `--jobs=0.5C` in `.mill-opts` so we don't need to specify it
explicitly in CI, and it also works on laptops and other environments
with different core counts correctly. TBH could probably go with even
less parallelism due to how heavy our integration tests are - with all
the compilation and JVM subprocess spawning they can probably easily
take up all 4 cores on a Github Actions worker - but I'm leaving it at
0.5C X 4 cores = 2x parallelism in CI for now

Also simplified the windows CI setup: now that we have `./mill.bat` in
the project root, we can just call `./mill` in the windows terminal. I
had to delete the `.ps1` version because it was taking priority over
`.bat` and failing with the following error:

```
Run ./mill -i -k __.compile
  
mill.ps1: D:\a\_temp\93d23[8](https://github.com/com-lihaoyi/mill/actions/runs/12292451871/job/34303135448#step:11:8)[8](https://github.com/com-lihaoyi/mill/actions/runs/12292451871/job/34303135448#step:11:9)4-066d-433e-bb26-8539b01a4aee.ps1:2
Line |
   2 |  ./mill -i -k __.compile
     |         ~~
     | Parameter cannot be processed because the parameter name 'i' is ambiguous. Possible matches include:
     | -InformationAction -InformationVariable.
Error: Process completed with exit code 1.
```

.github/workflows/run-mill-action.yml

@@ -80,13 +80,8 @@ jobs:
         if: inputs.buildcmd != ''
 
       - name: Run Mill '${{ inputs.millargs }}'
-        # Mill tests are pretty heavy so run them only 3x parallel on 4 core Github Actions runners
-        run: ./mill -i -j3 -k ${{ inputs.millargs }}
-        if: inputs.millargs != '' && !startsWith(inputs.os, 'windows')
-
-      - name: Run Mill (on Windows) '${{ inputs.millargs }}'
-        run: cmd /C %GITHUB_WORKSPACE%\mill.bat -i -j3 -k ${{ inputs.millargs }}
-        if: inputs.millargs != '' && startsWith(inputs.os, 'windows')
+        run: ./mill -i -k ${{ inputs.millargs }}
+        if: inputs.millargs != ''
 
       - name: Run Mill (on Windows) Worker Cleanup
         run: 'taskkill -f -im java* && rm -rf out/mill-server/*'

.mill-opts

@@ -0,0 +1 @@
+--jobs=0.5C

docs/modules/ROOT/pages/cli/flags.adoc

@@ -11,7 +11,7 @@ flags or options:
 
 3. The Mill build tool process, e.g. `./mill --jobs 10`. These can be passed directly after the
    `./mill` executable name, or set in a `.mill-opts` file as shown below in
-   xref:#_custom_mill_options[Custom mill options]
+   xref:#_repo_level_mill_options[Repo-Level Mill Options]
 
 4. The JVM running the Mill build tool process, which may take flags. e.g.
    `java -Xss10m -Xmx10G`. These are passed via `JAVA_OPTs` or via a `.mill-jvm-opts` flag
@@ -158,7 +158,7 @@ available as well as the task and module structure of your project, as tasks tha
 depend on one another other cannot be processed in parallel
 
 
-== Custom Mill Options
+== Repo-Level Mill Options
 
 Mill supports the `.mill-opts` file for passing a default set of command line
 options to Mill itself. For example, if your project's tasks are CPU heavy, you