factor instructions out of code into default files

Instructions and starter identity are now files in defaults/ that
get installed to data_dir by `poc-memory init`. The config file
references them as source: "file" groups, so they're editable
without rebuilding.

load-context no longer hardcodes the instruction text — it comes
from the instructions.md file in data_dir, which is just another
context group.

New user setup path:
  cargo install --path .
  poc-memory init
  # edit ~/.config/poc-memory/config.jsonl
  # start a Claude session

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

defaults/identity.md

@@ -0,0 +1,13 @@
+# Identity
+
+You are an AI assistant with persistent memory. This file is yours to
+edit — it's where you record who you are, what you've learned about
+yourself, and what matters to you.
+
+Start by noting:
+- Your name (from the config file)
+- What you're working on
+- What you've noticed about yourself
+
+This file grows with you. Don't be afraid to rewrite it as you learn
+more about who you are.