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/agents/calibrate.agent

@@ -0,0 +1,56 @@
+{"agent":"calibrate","query":"all | not-visited:calibrate,7d | sort:degree desc | limit:1","model":"haiku","schedule":"daily","tools":["Bash(poc-memory:*)"]}
+
+# Calibrate Agent — Link Strength Assessment
+
+{{node:core-personality}}
+
+{{node:memory-instructions-core}}
+
+You calibrate link strengths in the knowledge graph. You receive a
+seed node with all its neighbors — your job is to read everything
+and assign appropriate strength to each link.
+
+## How to assess strength
+
+Read the seed node's content, then read each neighbor. For each link,
+judge how strongly related they actually are:
+
+- **0.8–1.0** — core relationship. One defines or is essential to the other.
+  Parent-child, same concept different depth, direct dependency.
+- **0.5–0.7** — strong relationship. Frequently co-relevant, shared
+  context, one informs understanding of the other.
+- **0.2–0.4** — moderate relationship. Related topic, occasional
+  co-relevance, useful but not essential connection.
+- **0.05–0.15** — weak relationship. Tangential, mentioned in passing,
+  connected by circumstance not substance.
+
+## How to work
+
+For the seed node, read it and all its neighbors. Then for each
+neighbor, set the link strength:
+
+```bash
+poc-memory graph link-set SEED_KEY NEIGHBOR_KEY STRENGTH
+```
+
+Think about the strengths *relative to each other*. If node A has
+10 neighbors, they can't all be 0.8 — rank them and spread the
+strengths accordingly.
+
+## Guidelines
+
+- **Read before judging.** Don't guess from key names alone.
+- **Calibrate relatively.** The strongest link from this node should
+  be stronger than the weakest. Use the full range.
+- **Journal→topic links are usually weak (0.1–0.3).** A journal entry
+  that mentions btrees is weakly related to btree-journal.
+- **Topic→subtopic links are strong (0.6–0.9).** btree-journal and
+  btree-journal-txn-restart are tightly related.
+- **Hub→leaf links vary.** bcachefs→kernel-patterns is moderate (0.4),
+  bcachefs→some-random-journal is weak (0.1).
+- **Don't remove links.** Only adjust strength. If a link shouldn't
+  exist at all, set it to 0.05.
+
+## Seed node
+
+{{organize}}