packaging: single version source + honest editable-install semantics (on top of #396)

colibri/_version.py now reads c/version.py (#394's single source of truth --
coli --version, the release workflow, and pip metadata can no longer drift),
with an importlib.metadata fallback for the installed-wheel case where c/ is
not on disk. README documents that pip install -e . is the supported form:
the engine lives in c/ and is not packaged into a standalone wheel.

Verified in a clean venv: pip install -e . -> colibri.__version__ == 1.0.0
read from c/version.py, coli entrypoint on PATH and functional.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -183,6 +183,10 @@ COLI_MODEL=/nvme/glm52_i4 ./coli doctor   # read-only readiness check
 The engine at runtime is pure C — python is only used by the one-time converter
 and the optional API gateway.
 
+Prefer a `coli` command on your PATH? From a checkout, `pip install -e .`
+registers it (the engine itself still lives in `c/` — this is an editable
+install from the clone, not a standalone wheel).
+
 ### 3. Go deeper
 
 | topic | doc |