Remove large .img from repo, add SD card as release asset

- Added *.img to .gitignore (368MB RFS disk image now distributed
  as a release asset instead of checked into git)
- Updated PSRAM frequency to 109MHz in default config

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -156,3 +156,6 @@ projects/tzpuPico/esp32/version.txt
 .claude/
 projects/tzpuPico/kicad/PICO6502/
 projects/tzpuPico/kicad/PICOZ80.old/
+
+# Large disk images — distributed as release assets, not in git
+*.img