feat(best-practices): pointer CLAUDE.md so agents know the library exists

The 237KB library was mounted at /workspace/best-practices/ in every
consuming ConfigMap but nothing in the prompt mentioned it (directory
targets never enter the context manifest). New ~90-word CLAUDE.md file
target enters the system prompt: points at INDEX.md, instructs reading
only task-relevant topics.

Claude-Session: https://claude.ac/code/session_012arAwUuBQrpnMzJ36mEVBk

harnesses/contexts/best-practices/v1/CLAUDE.md

@@ -0,0 +1,7 @@
+# Best Practices Library
+
+A library of cross-project best-practices docs is mounted at `/workspace/best-practices/`.
+
+- Start with `/workspace/best-practices/INDEX.md` — one line per topic saying when to read it.
+- Read ONLY the topic file(s) relevant to your current task (e.g. `test-driven-development.md` for coding against tests, `api-design.md` for HTTP API work, `scripting.md` for shell scripts).
+- Do not read the whole library; each file is 10-60KB. Pick from the index.

harnesses/contexts/best-practices/v1/harness.yaml

@@ -6,5 +6,10 @@ requires: []
 provides: [best-practices]
 
 context_files:
+  # CLAUDE.md is a file target -> enters the context manifest (system prompt),
+  # pointing the agent at the mounted library below (directory targets are
+  # mount-only and never enter the prompt).
+  - source: ./CLAUDE.md
+    target: /opt/harness/context/best-practices/CLAUDE.md
   - source: ./best-practices/
     target: /workspace/best-practices/