Add CONTEXT.md index and context-loading task

First use of the CONTEXT.md pattern — thin index pointing to
context/ topic files, loaded by context-load at session start.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CONTEXT.md

@@ -0,0 +1,7 @@
+# Context — claude-foundations
+
+Current work focus and active tasks. Used by `context-load` to orient agents at session start.
+
+## Active
+
+- [Context loading improvements](context/context-loading.md) — Testing and refining the `context-load` and `start-claude` scripts