M1.6: document native command-file execution

Author: pgousdal

docs/M1_6_COMMAND_FILE_EXECUTION.md

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+# M1.6 Command-File Execution
+
+M1.6 adds basic AmigaDOS command-file execution without introducing an AmShell script language.
+
+## Interface
+
+A single non-option argument is treated as an AmigaDOS command file:
+
+```text
+AmShell scriptfile
+```
+
+AmShell delegates execution to the native AmigaDOS `EXECUTE` command through the existing system-Shell backend. The script is therefore interpreted by AmigaDOS, not by AmShell.
+
+## Compatibility rationale
+
+`EXECUTE` is the established AmigaDOS mechanism for command files. It provides native script semantics, including dot commands and argument substitution, and executes script lines as Shell commands.
+
+AmShell deliberately does **not**:
+
+- parse command-file lines itself;
+- invent a new script syntax;
+- translate scripts to POSIX/Bash syntax;
+- reinterpret AmigaDOS quoting, redirection, pipes, `FAILAT`, or script directives.
+
+The command-file path is quoted before it is passed to `EXECUTE`, including AmigaDOS quote escaping for a literal quote in a path.
+
+## M1.6 scope
+
+The first implementation covers a single command-file path without script arguments. This is intentionally narrow. Argument-bearing `EXECUTE` semantics (`.KEY`, keyword arguments, defaults and substitutions) require their own differential runtime qualification before AmShell exposes them through its command-line interface.
+
+The baseline fixture is:
+
+```text
+tests/compat/scripts/basic.script
+```
+
+## Qualification status
+
+M1.6 implementation and host repository checks may pass independently of runtime qualification. A native-vs-AmShell command-file differential run on AmigaOS/FS-UAE is still required before claiming full M1.6 runtime PASS.