fix: derive kirby_sheet.__version__ instead of restating it

It said "0.1.0" while pyproject said 0.2.0, so 0.2.0 shipped to PyPI with a
module attribute reporting the previous release. The distribution metadata was
right; only the literal was wrong, which is the worst shape -- pip reports one
number and the code reports another.

kirby-cost and kirby-combat already derive it from the installed distribution,
kirby-combat's comment noting the same drift bit it at 0.3.28. This is the
third instance, so it is the pattern rather than an accident.

229 passed, 0 skipped, oracle configured.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xof4ZUS5n6A5ibX3PXNYbs

Author: site-shell

kirby_sheet/__init__.py

@@ -8,4 +8,15 @@
 It ships no Hero Games content. Point it at a template you already have.
 """
 
-__version__ = "0.1.0"
+from importlib.metadata import PackageNotFoundError, version as _version
+
+try:
+    #: Read from the installed distribution rather than restated here.
+    #: A hardcoded literal drifts, and this one did: it still said "0.1.0"
+    #: when pyproject said 0.2.0 and the wheel published to PyPI as 0.2.0, so
+    #: anything reading kirby_sheet.__version__ got the wrong answer off a
+    #: correctly-built package. kirby-cost and kirby-combat already derive it
+    #: for exactly this reason.
+    __version__ = _version("kirby-sheet")
+except PackageNotFoundError:  # not installed (e.g. a source checkout on sys.path)
+    __version__ = "0.0.0+unknown"