docs: delete agent-cost; rename kb-search skill to brain (#14)

* docs: delete agent-cost; rename kb-search skill to brain.

bin/agents/cost does not exist. CI unittest now fails if a SKILL.md names a
missing bin/ path.

* test: gate SKILL.md bin/ paths; name the brain skill brain.

Follow-up to the agent-cost delete: unittest fails if a skill names a missing
tool. Frontmatter name is brain, not kb-search.

skills/brain/SKILL.md

@@ -0,0 +1,46 @@
+---
+name: brain
+description: >-
+  Deduction search over the 2dph brain (Ladybug graph: ops corpus, portfolio,
+  ssh hosts) with bin/brain/search.go instead of reading files or grepping
+  repos. Use whenever a question starts with "where is", "what runs on",
+  "which file describes", "who is", "how is X done", before opening any
+  documentation.
+---
+
+# brain — deduction over facts and info
+
+One embedded Ladybug graph (`var/kb.lbug`, read-only when queried) holding two
+roots:
+
+- **facts** — assertions backed by ≥2 independent sources (docker ps × compose
+  × ssh-config × docs), `confidence: confirmed`.
+- **info** — descriptive/narrative leafs, searchable, never asserted.
+
+Search = deduction: facts root first, info root second, `web-search` as the
+second independent source when local roots cannot confirm. An answer is
+`confirmed` only if it comes off the facts root; anything else is
+`(not confirmed)`.
+
+```bash
+bin/brain/search.go "Matrix federation"                # pointers + snippets, YAML
+bin/brain/search.go "onlyoffice postgres" --root facts # restrict to confirmed
+bin/brain/search.go "where is cs-lexicon" --json | yq '.[].ref'
+bin/brain/get.go <id> --body                           # full chunk only when needed
+bin/brain/stats.go                                     # index health
+bin/brain/eval.go                                      # recall@5 >= 0.95 gate
+```
+
+`bin/kb/search` is a deprecated wrapper. `--hop` errors (File/FROM_FILE edges
+are not wired yet); do not treat it as a graph walk.
+
+## Rules
+
+- Search before you read. Never grep a repo for a concept the graph covers.
+- `--root facts` returns only confirmed evidence-linked answers. Default shows
+  facts first, then info leafs clearly marked `(not confirmed)`.
+- If recall looks wrong, run `bin/brain/eval.go`; it gates control questions and
+  should stay at or above 95% recall@5.
+- Escalate to `web-search` (the `web-search` skill) as the independent second
+  source when both local roots cannot confirm; never report an unconfirmed
+  single-source local answer as fact.