feat: add TOML codec and envc format support

New toml package (pelletier/go-toml/v2) matching yaml/json options and merge flow.
envc convert/get/merge accept --from/--to toml; fixture and CLI tests; docs refresh.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/envc/doc.go

@@ -1,3 +1,3 @@
 // Command envc converts, queries, and merges configuration between env, YAML,
-// JSON, and INI formats.
+// JSON, TOML, and INI formats.
 package main