track the architecture docs

They were written outside git, so the reasoning that produces the design
had no history and no way to be superseded. capabilities.md and
geometry-vs-code.md are both known stale at this commit; they are tracked
as-is so the corrections are visible as movement rather than as a rewrite.

.gitignore

@@ -4,3 +4,6 @@ peripheral/.consent.json
 peripheral/.resume.json
 peripheral/.engram/
 peripheral/organ
+
+# Claude Code session state
+.claude/