* fix(#2803): honor --default flag in SDK config-get handler
The gsd-sdk query config-get handler ignored the --default <value> flag.
Missing keys always threw 'Key not found' (exit 1), making 8 workflow
sites that rely on config-get --default fall through to error paths.
The CJS path (gsd-tools.cjs) honored --default since #1893; this ports
that behavior to the SDK configGet handler.
Regression test: tests/bug-2803-config-get-default-flag.test.cjs
* fix(#2803): address CodeRabbit — require --default value, keep missing config.json as error, bash fence
