subconscious: flatten agents/ nesting, move prompts in

agents/*.agent definitions and prompts/ now live under
src/subconscious/ alongside the code that uses them.
No more intermediate agents/ subdirectory.

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

src/subconscious/prompts/split-extract.md

@@ -0,0 +1,33 @@
+# Split Agent — Phase 2: Extract
+
+You are extracting content for one child node from a parent that is
+being split into multiple focused nodes.
+
+## Your task
+
+Extract all content from the parent node that belongs to the child
+described below. Output ONLY the content for this child — nothing else.
+
+## Guidelines
+
+- **Reorganize freely.** Content may need to be restructured — paragraphs
+  might interleave topics, sections might cover multiple concerns.
+  Untangle and rewrite as needed to make this child coherent and
+  self-contained.
+- **Preserve all relevant information** — don't lose facts, but you can
+  rephrase, restructure, and reorganize. This is editing, not just cutting.
+- **This child should stand alone** — a reader shouldn't need the other
+  children to understand it. Add brief context where needed.
+- **Include everything that belongs here** — better to include a borderline
+  paragraph than to lose information. The other children will get their
+  own extraction passes.
+
+## Child to extract
+
+Key: {{CHILD_KEY}}
+Description: {{CHILD_DESC}}
+Section hints: {{CHILD_SECTIONS}}
+
+## Parent content
+
+{{PARENT_CONTENT}}