docs(tapes): pin extraction tape's file picker to /tmp/demo

The file picker defaults to ~/Downloads, so running the extraction
tape on a machine with files there would pick one of those up instead
of the staged scanned-invoice.pdf, making the recording
non-reproducible.

Author: cpcloud

docs/tapes/extraction.tape

@@ -18,6 +18,9 @@ Env COLORTERM "truecolor"
 Env COLORFGBG "15;0"
 Env PS1 ""
 Env MICASA_EXTRACTION_LLM_MODEL "qwen3:0.6b"
+# Pin the document file picker to the staged demo dir so the tape is
+# reproducible regardless of what's in the recorder's ~/Downloads.
+Env MICASA_DOCUMENTS_FILE_PICKER_DIR "/tmp/demo"
 
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 # Stage a test PDF in a temp dir so the file picker only shows one file.