agents: self-contained agent files with embedded prompts

Each agent is a .agent file: JSON config on the first line, blank line,
then the raw prompt markdown. Fully self-contained, fully readable.
No separate template files needed.

Agents dir: checked into repo at poc-memory/agents/. Code looks there
first (via CARGO_MANIFEST_DIR), falls back to ~/.claude/memory/agents/.

Three agents migrated: replay, linker, transfer.

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

poc-memory/src/agents/mod.rs

@@ -18,6 +18,7 @@
 pub mod transcript;
 pub mod llm;
 pub mod prompts;
+pub mod defs;
 pub mod audit;
 pub mod consolidate;
 pub mod knowledge;