Bump Hayabusa 2.17.0→3.8.1, Takajo 2.6.0→2.15.1; fix takajo CWD

Upstream renamed release assets (linux-intel → lin-x64-gnu,
linux-arm → lin-aarch64-gnu); updated accordingly.

Takajo was silently failing because start.sh invoked it from
WORKDIR=/data — takajo checks for companion files in CWD and
exits with "The Takajo executable does not exist in the current
directory." Wrap the call in (cd /opt/hayabusa && ./takajo ...)
so automagic reports actually land in /output/takajo/.

Also add .dockerignore to keep test-data/ and .git/ out of the
build context.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

start.sh

@@ -38,7 +38,7 @@ echo "Output is going to: ${output}"
 
 # Check if takajo exists before running it
 if [ -x /opt/hayabusa/takajo ]; then
-    /opt/hayabusa/takajo automagic -t "${output}_takajo.jsonl" -o "${outdir}/takajo"
+    ( cd /opt/hayabusa && ./takajo automagic -t "${output}_takajo.jsonl" -o "${outdir}/takajo" )
 else
     echo "[!] Takajo is not available on this platform."
 fi