fix(source): wrap only JSON parse errors in readJson

[SebTardif]

What Problem This Solves

readJson caught every failure from readFileSync and JSON.parse and rethrew it as malformed JSON. A directory whose name ends in .json, a permission error, or a file that vanishes after existsSync all looked like a parse bug. Parse failures should stay labeled that way. I/O failures should keep their original code.

Evidence

Live node import of readJson on the patched tree. A path that is a directory throws EISDIR with no malformed JSON text. Truncated JSON still throws the wrapped parse message.

$ node --input-type=module -e "import { readJson } from './scripts/source.mjs'; ..."
DIR_NAME Error
DIR_CODE EISDIR
DIR_MSG EISDIR: illegal operation on a directory, read
PARSE_NAME Error
PARSE_MSG [clawsweeper-state] readJson: malformed JSON in C:\Users\sebta\AppData\Local\Temp\proof-json-pKZBgI\bad.json

Real behavior proof

• Behavior or issue addressed: Directory and other I/O failures from readJson keep their original Node code. Only JSON.parse SyntaxError is wrapped as malformed JSON.
• Real environment tested: Windows 11, Node v24.19.0, patched scripts/source.mjs on branch fix/readjson-syntaxerror-only.
• Exact steps or command run after this patch: Created a temp path run.json as a directory and a sibling bad.json with truncated JSON. Imported readJson from ./scripts/source.mjs with node --input-type=module and called it on both paths.
• Evidence after fix: terminal output from that node command (see Evidence). The directory path prints EISDIR and illegal operation on a directory. The truncated file prints the wrapped malformed JSON message.
• Observed result after fix: The catch in readJson rethrows non-SyntaxError values unchanged, so operators see EISDIR instead of a fake parse failure.
• What was not tested: EACCES on a locked file.

Change

scripts/source.mjs wraps SyntaxError only.

Origin

Introduced in 7964f2af5f7.