jrag: reject --count + --exists together (argparse mutex)

[HumanBean17]

Surfaced by the 4-reviewer fan-out on PR #422 (issue #376). Deferred as a Minor — current behavior is correct and documented.

Current behavior

--count and --exists are both accepted together (both live on _common_parser). --exists wins via the documented precedence chain in render() (exists > count > fields > detail) and _emit's exit-code check:

$ jrag find Foo --count --exists
true          # --exists output; exit 0/2 on hit/miss

No warning, no error — a user who passes both almost certainly meant only one.

Proposal

Make them mutually exclusive at parse time:

shape = common.add_mutually_exclusive_group()
shape.add_argument("--count", action="store_true", ...)
shape.add_argument("--exists", action="store_true", ...)

Related

--fields is also a no-op when combined with --count/--exists (it shapes node projection, which those bypass). As of #422 that's documented in --help and the skill/agent docs, but not enforced. A mutex can't cover --fields (it's a different axis — a normal-projection override), so it stays a documented no-op.

Priority

Low — defer unless a user actually trips over it. Filing so it's tracked.