docs: avoid hard-coded Scala versions in docs

Motivation:
The development docs embedded concrete Scala cross versions, which drift when build.mill is updated and create recurring maintenance work.

Modification:
Point CLAUDE.md at build.mill as the source of truth for Mill and Scala versions, replace concrete JVM cross-version commands with Mill wildcard targets, and update README development/client-server examples to use the current cross-module syntax without pinning a Scala version.

Result:
The docs now stay aligned with the configured build matrix without requiring manual version edits when Scala versions change.

Author: He-Pin

CLAUDE.md

@@ -2,26 +2,26 @@
 
 ## Build & Test
 
-Uses [Mill](https://mill-build.org/) 1.1.2. Cross-built for Scala 3.3.7, 2.13.18, and 2.12.21 (JVM only).
+Uses the [Mill](https://mill-build.org/) version declared in `build.mill`. Cross-built for the Scala versions declared by `scalaVersions` and `jvmScalaVersions` in `build.mill`.
 
 ```bash
-# Compile (JVM, Scala 3)
-./mill 'sjsonnet.jvm[3.3.7]'.compile
+# Compile all JVM Scala versions
+./mill 'sjsonnet.jvm[_]'.compile
 
-# Run all JVM tests (Scala 3)
-./mill 'sjsonnet.jvm[3.3.7]'.test
+# Run all JVM tests
+./mill 'sjsonnet.jvm[_]'.test
 
 # Run all tests across all platforms
 ./mill __.test
 
-# Build assembly JAR
-./mill 'sjsonnet.jvm[3.3.7]'.assembly
+# Build assembly JARs
+./mill 'sjsonnet.jvm[_]'.assembly
 
 # GraalVM native image
 ./mill sjsonnet.graal.nativeImage
 ```
 
-The assembly JAR is at `out/sjsonnet/jvm/3.3.7/assembly.dest/out.jar`. Run it with `java -Xss100m -jar out.jar`.
+Assembly JARs are written under `out/sjsonnet/jvm/<scala-version>/assembly.dest/out.jar`. Run one with `java -Xss100m -jar out.jar`.
 
 ## Formatting
 
@@ -71,7 +71,7 @@ Every bug fix should include a regression test:
 2. Create `new_test_suite/<descriptive_name>.jsonnet.golden` with expected output.
    - Success tests: JSON output followed by a newline, using `std.assertEqual` chains ending in `true`.
    - Error tests (filename starts with `error.`): expected stderr including stack traces.
-3. Run `./mill 'sjsonnet.jvm[3.3.7]'.test` to verify.
+3. Run `./mill 'sjsonnet.jvm[_]'.test` to verify.
 
 ## Debug Stats
 

readme.md

@@ -335,16 +335,16 @@ To generate the static files without starting a server:
 The output is a single `index.html` file (with the Sjsonnet JS engine inlined) written to `out/playground/bundle.dest/`. You can open it directly in a browser — no server needed — or serve it with any static file server. The editor UI (CodeMirror) is loaded from a CDN at runtime, so an internet connection is required.
 
 ## Development
-To compile the JVM version:
+To compile the JVM versions:
 
 ```bash
-./mill 'sjsonnet.jvm[3.3.7]'.compile
+./mill 'sjsonnet.jvm[_]'.compile
 ```
 
-To run the tests:
+To run the JVM tests:
 
 ```bash
-./mill 'sjsonnet.jvm[3.3.7]'.test
+./mill 'sjsonnet.jvm[_]'.test
 ```
 
 If you want to test for all platforms and versions, you can run
@@ -357,16 +357,16 @@ If you want to test for all platforms and versions, you can run
 
 Sjsonnet comes with a built in thin-client and background server, to help
 mitigate the unfortunate JVM warmup overhead that adds ~1s to every invocation
-down to 0.2-0.3s. For the simple non-client-server executable, you can use
+down to 0.2-0.3s. For the simple non-client-server executables, you can use
 
 ```bash
-./mill -i show sjsonnet[3.3.7].jvm.assembly
+./mill -i show 'sjsonnet.jvm[_].assembly'
 ```
 
-To create the executable. For the client-server executable, you can use
+For the client-server executables, you can use
 
 ```bash
-./mill -i show sjsonnet[3.3.7].server.assembly
+./mill -i show 'sjsonnet.jvm[_].server.assembly'
 ```
 
 By default, the Sjsonnet background server lives in `~/.sjsonnet`, and lasts 5