Initial DOS VM tooling setup

.pi/extensions/dos-vm.md

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+# DOS VM tools
+
+This project-local Pi extension exposes `dos_read`, `dos_write`, `dos_edit`,
+`dos_list`, and `dos_exec`. It communicates with `DOSAGENT.EXE` over COM1
+through the serial relay controller port at `127.0.0.1:5555`.
+
+The agent is not started automatically. From the FreeDOS VGA console, run
+`DOSAGENT` (installed in `C:\\FREEDOS\\BIN`, which is on `PATH`) when Pi tool
+access is needed. While it runs, press Ctrl+C in the VGA console to stop it and
+return to the DOS prompt; Pi tools then cannot connect until it is started
+again. QEMU monitor control remains available at `127.0.0.1:4444`.
+
+Connect PuTTY in Raw mode to `127.0.0.1:5557` for a read-only decoded mirror of
+Pi/DOS serial traffic. Port 5556 is reserved for the internal QEMU-to-relay
+connection.
+
+`D:` is QEMU's FAT-directory view of `.qemu\\share`. Treat it as an exchange
+volume, not a live-sync mount: host-side changes can be stale or partially
+visible to a running VM. Restart QEMU before compiling host-edited files, or
+use `dos_write` to place the source on `C:` before compiling.