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

.claude/settings.json

@@ -22,6 +22,16 @@
             "type": "command",
             "command": "node \"$CLAUDE_PROJECT_DIR/.wolf/hooks/post-write.js\"",
             "timeout": 10
+          },
+          {
+            "type": "command",
+            "command": "FILE_PATH=$(python3 -c \"import json,sys; d=json.load(sys.stdin); print(d.get('tool_input',{}).get('file_path') or d.get('tool_input',{}).get('path') or '')\" 2>/dev/null); [[ \"$FILE_PATH\" == *.py ]] && ruff format --quiet \"$FILE_PATH\" 2>/dev/null; true",
+            "timeout": 10
+          },
+          {
+            "type": "command",
+            "command": "FILE_PATH=$(python3 -c \"import json,sys; d=json.load(sys.stdin); print(d.get('tool_input',{}).get('file_path') or d.get('tool_input',{}).get('path') or '')\" 2>/dev/null); [[ \"$FILE_PATH\" == *.py ]] && ruff check --quiet \"$FILE_PATH\" 2>/dev/null; true",
+            "timeout": 10
           }
         ]
       }