Add calibrate agent, link-set command, and dominating-set query stage

calibrate.agent: Haiku-based agent that reads a node and all its
neighbors, then assigns appropriate link strengths relative to each
other. Designed for high-volume runs across the whole graph.

graph link-set: Set strength of an existing link (0.0-1.0).

dominating-set query stage: Greedy 3-covering dominating set — finds
the minimum set of nodes such that every node in the input is within
1 hop of at least 3 selected nodes. Use with calibrate agent to
ensure every link gets assessed from multiple perspectives.

Usage: poc-memory query "content ~ 'bcachefs' | dominating-set"

poc-memory/src/cli/graph.rs

@@ -178,6 +178,35 @@ pub fn cmd_link_add(source: &str, target: &str, reason: &[String]) -> Result<(),
     Ok(())
 }
 
+pub fn cmd_link_set(source: &str, target: &str, strength: f32) -> Result<(), String> {
+    super::check_dry_run();
+    let mut store = store::Store::load()?;
+    let source = store.resolve_key(source)?;
+    let target = store.resolve_key(target)?;
+    let strength = strength.clamp(0.01, 1.0);
+
+    let mut found = false;
+    for rel in &mut store.relations {
+        if rel.deleted { continue; }
+        if (rel.source_key == source && rel.target_key == target)
+            || (rel.source_key == target && rel.target_key == source)
+        {
+            let old = rel.strength;
+            rel.strength = strength;
+            println!("Set: {} ↔ {} strength {:.2} → {:.2}", source, target, old, strength);
+            found = true;
+            break;
+        }
+    }
+
+    if !found {
+        return Err(format!("No link found between {} and {}", source, target));
+    }
+
+    store.save()?;
+    Ok(())
+}
+
 pub fn cmd_link_impact(source: &str, target: &str) -> Result<(), String> {
     let store = store::Store::load()?;
     let source = store.resolve_key(source)?;