split agent: two-phase node decomposition for memory consolidation

Phase 1 sends a large node with its neighbor communities to the LLM
and gets back a JSON split plan (child keys, descriptions, section
hints). Phase 2 fires one extraction call per child in parallel —
each gets the full parent content and extracts/reorganizes just its
portion.

This handles arbitrarily large nodes because output is always
proportional to one child, not the whole parent. Tested on the kent
node (19K chars → 3 children totaling 20K chars with clean topic
separation).

New files:
  prompts/split-plan.md   — phase 1 planning prompt
  prompts/split-extract.md — phase 2 extraction prompt
  prompts/split.md        — original single-phase (kept for reference)

Modified:
  agents/prompts.rs — split_candidates(), split_plan_prompt(),
                      split_extract_prompt(), agent_prompt "split" arm
  agents/daemon.rs  — job_split_agent() two-phase implementation,
                      RPC dispatch for "split" agent type
  tui.rs            — added "split" to AGENT_TYPES

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}}