chore: gitignore encrypted scratchpad

The scratchpad is encrypted with a per-user key (already gitignored).
The encrypted blob is personal data with no value to other
contributors and creates noise in diffs.

Spec: specs/ast-audit-tests.md

Signed-off-by: Jose Alekhinne <jose@ctx.ist>

Author: josealekhine

.context/scratchpad.enc

@@ -71,6 +71,7 @@ editors/vscode/*.vsix
 ideas
 .context/.ctx.key
 .context/.scratchpad.key
+.context/scratchpad.enc
 .DS_Store
 .gitnexus