fix(agent): parse single-line fenced JSON in _parse_json

[AmirF194]

_parse_json split a fenced response on newlines to strip the ``` code fence:

lines = text.splitlines()
inner = lines[1:-1] if lines[-1].strip() == "```" else lines[1:]
text = "\n".join(inner).lstrip("json").strip()

When the opening fence, language tag, and JSON share one line (e.g. ```json{"site": "MAIN"}```, no internal newline), text.splitlines() returns a single-element list. lines[-1].strip() == "```" is then false, so inner = lines[1:] is [], and text becomes an empty string before json.loads or the brace-scanning fallback ever run. Valid JSON is discarded and _parse_json returns None; the caller (_resolve_args_with_llm) treats that as unparseable and calls the tool with no arguments instead of the arguments the LLM produced.

Replaced the split/join with a single regex that matches the fence and optional language tag whether or not they share a line with the payload, so both layouts extract the same content.

Verified

Docker (python:3.12-slim, editable install, python3 -c 'import agent.plan_execute.executor as m; print(m.__file__)' confirmed the real repo module, not a stale install):

• pytest src/agent/tests/ on this branch: 74 passed, including the two new regression tests.
• Reverting only executor.py to HEAD (e5dc83b) while keeping the new tests: pytest -k single_line_fence fails with assert None == {'site': 'MAIN'} on both, confirming they fail on main and pass on the fix.
• ruff check / ruff format --check on both changed files: clean. The 6 pre-existing ruff check findings elsewhere in executor.py (import order, an undefined forward-ref type, two SIM117 nested-with suggestions, and a removeprefix suggestion in the unrelated _parse_tool_call helper) are unchanged from HEAD and outside this diff's scope.

Not touched: _parse_tool_call a few lines below has the same fence-splitting shape and would show the same bug on a single-line input, but its own docstring says it's "utility, not used in main path," so it isn't part of this fix.

Fixes #482