Add codespaces skill for belief-map architecture discovery

Initial import of the codespaces skill: query .belief_map.sexp for module
boundaries, dependencies, blast radius, and the minimal files to read before
non-trivial code changes.

Includes the belief-map and infra-topology builders, the belief-search query
engine and its tests, and reference docs for the S-expression notation and
query patterns.

.gitignore

@@ -0,0 +1,11 @@
+# macOS
+.DS_Store
+
+# Python
+__pycache__/
+*.py[cod]
+.ruff_cache/
+
+# Generated belief-map artifacts (produced by scripts/build_belief_map.py)
+.belief_map.sexp
+.belief_map_cache.json