agents: neighborhood placeholder, organize prompt, weight-set command

Add {{neighborhood}} placeholder for agent prompts: full seed node
content + ranked neighbors (score = link_strength * node_weight) with
smooth cutoff, minimum 10, cap 25, plus cross-links between included
neighbors.

Rewrite organize.agent prompt to focus on structural graph work:
merging duplicates, superseding junk, calibrating weights, creating
concept hubs.

Add weight-set CLI command for direct node weight manipulation.

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

poc-memory/src/main.rs

@@ -160,6 +160,14 @@ EXAMPLES:
         /// Node key
         key: String,
     },
+    /// Set a node's weight directly
+    #[command(name = "weight-set")]
+    WeightSet {
+        /// Node key
+        key: String,
+        /// Weight (0.01 to 1.0)
+        weight: f32,
+    },
     /// Record a gap in memory coverage
     Gap {
         /// Gap description
@@ -769,6 +777,7 @@ fn main() {
         Command::Wrong { key, context } => cli::node::cmd_wrong(&key, &context),
         Command::NotRelevant { key } => cli::node::cmd_not_relevant(&key),
         Command::NotUseful { key } => cli::node::cmd_not_useful(&key),
+        Command::WeightSet { key, weight } => cli::node::cmd_weight_set(&key, weight),
         Command::Gap { description } => cli::node::cmd_gap(&description),
 
         // Node