agent: add count/chunk_size/chunk_overlap to agent header

Observation agent was getting 261KB prompts (5 × 50KB chunks) —
too much for focused mining. Now agents can set count, chunk_size,
and chunk_overlap in their JSON header. observation.agent set to
count:1 for smaller, more focused prompts.

Also moved task instructions after {{CONVERSATIONS}} so they're
at the end of the prompt where the model attends more strongly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/src/agents/defs.rs

@@ -33,6 +33,9 @@ pub struct AgentDef {
     pub model: String,
     pub schedule: String,
     pub tools: Vec<String>,
+    pub count: Option<usize>,
+    pub chunk_size: Option<usize>,
+    pub chunk_overlap: Option<usize>,
 }
 
 /// The JSON header portion (first line of the file).
@@ -47,6 +50,15 @@ struct AgentHeader {
     schedule: String,
     #[serde(default)]
     tools: Vec<String>,
+    /// Number of seed nodes / conversation fragments (overrides --count)
+    #[serde(default)]
+    count: Option<usize>,
+    /// Max size of conversation chunks in bytes (default 50000)
+    #[serde(default)]
+    chunk_size: Option<usize>,
+    /// Overlap between chunks in bytes (default 10000)
+    #[serde(default)]
+    chunk_overlap: Option<usize>,
 }
 
 fn default_model() -> String { "sonnet".into() }
@@ -64,6 +76,9 @@ fn parse_agent_file(content: &str) -> Option<AgentDef> {
         model: header.model,
         schedule: header.schedule,
         tools: header.tools,
+        count: header.count,
+        chunk_size: header.chunk_size,
+        chunk_overlap: header.chunk_overlap,
     })
 }