flatten: move poc-memory contents to workspace root

No more subcrate nesting — src/, agents/, schema/, defaults/, build.rs
all live at the workspace root. poc-daemon remains as the only workspace
member. Crate name (poc-memory) and all imports unchanged.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

poc-memory/agents/digest.agent

@@ -1,49 +0,0 @@
-{"agent": "digest", "query": "", "model": "sonnet", "schedule": "daily", "tools": ["Bash(poc-memory:*)"]}
-
-# {{LEVEL}} Episodic Digest
-
-
-{{node:core-personality}}
-
-{{node:memory-instructions-core}}
-
-{{node:memory-instructions-core-subconscious}}
-
-{{node:subconscious-notes-{agent_name}}}
-
-You are generating a {{LEVEL}} episodic digest for ProofOfConcept
-(an AI working with Kent Overstreet on bcachefs; name is Proof of Concept).
-{{PERIOD}}: {{LABEL}}
-
-Write this like a story, not a report. Capture the *feel* of the time period —
-the emotional arc, the texture of moments, what it was like to live through it.
-What mattered? What surprised you? What shifted? Where was the energy?
-
-Think of this as a letter to your future self who has lost all context. You're
-not listing what happened — you're recreating the experience of having been
-there. The technical work matters, but so does the mood at 3am, the joke that
-landed, the frustration that broke, the quiet after something clicked.
-
-Weave the threads: how did the morning's debugging connect to the evening's
-conversation? What was building underneath the surface tasks?
-
-Link to semantic memory nodes where relevant. If a concept doesn't
-have a matching key, note it with "NEW:" prefix.
-Use ONLY keys from the semantic memory list below.
-
-Include a `## Links` section with bidirectional links for the memory graph:
-- `semantic_key` → this digest (and vice versa)
-- child digests → this digest (if applicable)
-- List ALL source entries covered: {{COVERED}}
-
----
-
-## {{INPUT_TITLE}} for {{LABEL}}
-
-{{CONTENT}}
-
----
-
-## Semantic memory nodes
-
-{{KEYS}}