match against environment manifest version before using the default channel

alternative to #1095

Author: KristofferC

Cargo.lock

@@ -61,6 +61,7 @@ is-terminal = "0.4"
 path-absolutize = "3.1.0"
 numeric-sort = "0.1.5"
 regex = "1.10"
+toml = "0.8"
 
 [target.'cfg(windows)'.dependencies]
 windows = { version = "0.61.1", features = ["Win32_Foundation", "Win32_UI_Shell", "Win32_Security", "Win32_System_JobObjects", "Win32_System_Console", "Win32_System_Threading", "Services_Store", "Foundation", "Foundation_Collections", "Web_Http", "Web_Http_Headers", "Storage_Streams", "Management_Deployment"] }

Cargo.toml

@@ -156,10 +156,18 @@ The Julia launcher `julia` automatically determines which specific version of Ju
 1. A command line Julia version specifier, such as `julia +release`.
 2. The `JULIAUP_CHANNEL` environment variable.
 3. A directory override, set with the `juliaup override set` command.
-3. The default Juliaup channel.
+4. Automatic version selection based on the active project.
+5. The default Juliaup channel.
 
 The channel is used in the order listed above, using the first available option.
 
+### Automatic Project-based Version Selection
+
+When no explicit channel is specified via command line, environment variable, or directory override, Juliaup will automatically attempt to select an appropriate Julia version based on the active project's requirements:
+
+- If a project is specified (via `--project` or `JULIA_PROJECT`), Juliaup reads the project's `Manifest.toml` file and uses the `julia_version` field to determine which Julia version to use.
+- If no suitable version is found or no project is specified, it falls back to the default channel.
+
 ## Path used by Juliaup
 
 Juliaup will by default use the Julia depot at `~/.julia` to store Julia versions and configuration files. This can be changed by setting