feat(envc): add support for converting current process environment to YAML

Enhance the `envc convert` command to allow conversion of the current process environment to YAML format. Update README with usage examples and clarify input options. Add tests to ensure correct functionality for environment variable handling.

README.md

@@ -85,6 +85,24 @@ b, _ := json.New().Marshal(m)
 os.WriteFile("out.json", b, 0o644)
 ```
 
+### 4. Current process environment → YAML (library)
+
+```go
+ctx := context.Background()
+m, err := env.New(env.WithCurrentEnvironment()).Map(ctx)
+if err != nil {
+	return err
+}
+b, err := yaml.New().Marshal(m)
+if err != nil {
+	return err
+}
+_, _ = os.Stdout.Write(b)
+```
+
+Same map as `envc convert --from env --to yaml --input environ` (nested keys come from
+splitting `VAR_NAME` on `_`, then **lower+alnum** normalization).
+
 ## CLI: `envc`
 
 Install the binary from [**Install**](#install) (`go install …/cmd/envc@latest`). Every
@@ -109,7 +127,16 @@ envc version
 ### `convert`
 
 One input → normalize keys → one output. Defaults: **`--input -`**, **`--output -`**
-(stdin / stdout).
+(stdin / stdout). Use **`--input environ`** with **`--from env`** to read **`os.Environ()`**
+(snapshot at run time), then emit **`--to`** (YAML, JSON, etc.) on stdout—no stdin.
+
+```bash
+# Current process environment → nested YAML on stdout (may include secrets; mask before sharing)
+envc convert --from env --to yaml --input environ --output -
+
+# Same snapshot as minified JSON
+envc convert --from env --to json --input environ
+```
 
 ```bash
 # Helm-style values file → JSON on the terminal (redirect to a file if you prefer)
@@ -208,6 +235,9 @@ set +a
 ### Stdin, URLs, and EOF
 
 ```bash
+# Process environment (not stdin): use --input environ with --from env (see `convert` above)
+envc convert --from env --to yaml --input environ
+
 # Explicit stdin redirect (reads until EOF)
 envc convert --from yaml --to json --input - --output - <./service.yaml