feat(admin-config-tool): task 1.3 - nix and xce-build.sh integration

Add python3 + pycapnp/pyyaml/jsonschema/click/pytest/ruff (+pip) to the
nix dev shell, wire a new `xceadmin` case into xce-build.sh (schema
sync from src/schema/ and src/configs/schema/, pip install by profile,
pytest), package-data config so bundled schemas survive a non-editable
install, xce-build SKILL.md docs, and ruff format/check hooks matching
this repo's stdin-JSON hook convention (not brief.md's env-var sketch,
which would have silently no-op'd).

Verified live end-to-end inside `nix develop ./src` for both the
default (editable) and release (pinned) profiles; found and fixed 3
real bugs along the way (missing pip, PEP 668 externally-managed-
environment, missing package-data for release-profile bundling) —
logged as bug-1458/1459/1460.

Requirements: 5.5, 8.1, 8.2, 8.6

src/nixSupport/xce.nix

@@ -61,6 +61,18 @@ stdenv.mkDerivation {
     }))
     packcc
     cacert
+    # Python — for xceadmin (admin config tool, src/xceadmin/). pycapnp links
+    # against the capnproto C++ runtime above; nix's python3Packages.pycapnp
+    # handles that link correctly, unlike a plain `pip install pycapnp`.
+    # See .kiro/specs/admin-config-tool/brief.md "Nix Environment".
+    python3
+    python3Packages.pip
+    python3Packages.pycapnp
+    python3Packages.pyyaml
+    python3Packages.jsonschema
+    python3Packages.click
+    python3Packages.pytest
+    python3Packages.ruff
   ]  ++ (if withDebug then [
     (librtti_debug.overrideAttrs (oldattrs: {
       separateDebugInfo = true;