[correctness/low] CLI --since parsing raises bare ValueError on malformed input across six index commands

[tomharris]

Six index commands parse --since with int(since.rstrip("d")) and no validation, at
cli.py:300, 330, 367, 397, 438, 471. Malformed input produces a raw ValueError traceback rather
than a CLI error. rstrip("d") only strips trailing ds, so plausible typos all crash:
--since 90days, --since 3w, --since abc, --since d.

Location: devrag/cli.py:300-471 (six call sites)

Suggested fix:

Extract one shared parser and call it from all six sites:

def _parse_since(since: str | None) -> int | None:
    """Parse a lookback period like '90d' into days."""
    if not since:
        return None
    m = re.fullmatch(r"(\d+)d", since.strip())
    if not m:
        raise typer.BadParameter(
            f"invalid --since value {since!r}; expected a day count like '90d'"
        )
    return int(m.group(1))

typer.BadParameter renders as a proper CLI usage error with exit code 2 instead of a traceback.
The regex also rejects the currently-accepted-but-odd 90dd. If other units are wanted later
(3w, 6m), this is the single place to extend.

Source: Surfaced by /engineer-agent audit-code (June run, cited line 300 only); re-verified by hand against HEAD d97b2eb on 2026-07-15 and scope corrected to six call sites, confidence high.