config: add environment variable macros (#466)

* config: add environment variable macros

Add support for ${env.VAR_NAME} syntax to pull values from system
environment variables during config loading.

- env macros processed before regular macros (allows macros to reference env vars)
- works in cmd, cmdStop, proxy, checkEndpoint, filters.stripParams, metadata
- returns error if env var is not set
- add comprehensive tests

fixes #462

* docs: add env macro example to config.example.yaml

config.example.yaml

@@ -90,6 +90,9 @@ apiKeys:
 # - macro names must not be a reserved name: PORT or MODEL_ID
 # - macro values can be numbers, bools, or strings
 # - macros can contain other macros, but they must be defined before they are used
+# - environment variables can be referenced with ${env.VAR_NAME} syntax
+#   - env macros are substituted first, before regular macros
+#   - if the env var is not set, config loading will fail with an error
 macros:
   # Example of a multi-line macro
   "latest-llama": >
@@ -102,6 +105,11 @@ macros:
   # but they must be previously declared.
   "default_args": "--ctx-size ${default_ctx}"
 
+  # Example of environment variable macros
+  # - ${env.VAR_NAME} pulls the value from the system environment
+  # - useful for paths, secrets, or machine-specific configuration
+  "models_dir": "${env.HOME}/models"
+
 # models: a dictionary of model configurations
 # - required
 # - each key is the model's ID, used in API requests