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.

PLAN.md

@@ -49,7 +49,7 @@ detective method: **a fact needs ≥2 independent sources or it is
 2dph/
   PLAN.md / AGENTS.md
   docs/                     published docs (this conversation → docs/ as md)
-  skills/                   in-project skills (web-search, db-yaml, kb-search, agent-cost, diataxis-docs, …)
+  skills/                   in-project skills (web-search, db-yaml, brain, diataxis-docs)
   bin/
     facts/extract           auto-pair 2 sources → lexicon yaml + graph
     facts/audit             ["self"|"facts"|"info"|"stale"] 2-source + staleness gate
@@ -140,7 +140,7 @@ Feedback loop: every commit → PR → CI → green/gate → merge. Same discipl
 
 1. scaffold repo (:done after this file + AGENTS.md + .gitignore + ci)
 2. gh repo create eSlider/2dph --private + initial commit + CI
-3. vendored skill integration (web-search, db-yaml, kb-search, agent-cost, diataxis-docs) — no remote links
+3. vendored skill integration (web-search, db-yaml, brain, diataxis-docs) — no remote links
 4. .venv: ladybug + model2vec + mistune
 5. schema + tools with TDD (kb + md + facts + brain)
 6. ~/.config/brain config

bin/tools/test_published_docs.py

@@ -43,7 +43,7 @@ class PublishedDocsTest(unittest.TestCase):
         paths = [
             ROOT / "README.md",
             ROOT / "docs" / "design.md",
-            ROOT / "skills" / "kb-search" / "SKILL.md",
+            ROOT / "skills" / "brain" / "SKILL.md",
             ROOT / "skills" / "diataxis-docs" / "SKILL.md",
         ]
         # Command-style `--hop 1` / `--hop N` plus follow/walk = the old lie.

bin/tools/test_skills_bin_paths.py

@@ -0,0 +1,33 @@
+"""Every bin/ path named in skills/ must exist on disk."""
+from __future__ import annotations
+
+import re
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+BIN_PATH = re.compile(r"\b(bin/[A-Za-z0-9_./-]+)")
+
+
+class SkillsBinPathsTest(unittest.TestCase):
+    def test_agent_cost_skill_is_gone(self) -> None:
+        self.assertFalse(
+            (ROOT / "skills" / "agent-cost").exists(),
+            "skills/agent-cost documents bin/agents/cost which does not exist",
+        )
+
+    def test_brain_skill_replaces_kb_search(self) -> None:
+        self.assertTrue((ROOT / "skills" / "brain" / "SKILL.md").is_file())
+        self.assertFalse((ROOT / "skills" / "kb-search").exists())
+
+    def test_skill_bin_paths_exist(self) -> None:
+        missing: list[str] = []
+        for skill in sorted((ROOT / "skills").rglob("SKILL.md")):
+            text = skill.read_text()
+            for match in BIN_PATH.findall(text):
+                rel = match.rstrip("`'.,")
+                if rel.endswith(".go") or Path(rel).suffix == "" or Path(rel).suffix in {".go", ".py"}:
+                    p = ROOT / rel
+                    if not p.exists():
+                        missing.append(f"{skill.relative_to(ROOT)}: {rel}")
+        self.assertEqual(missing, [], "SKILL.md names bin/ paths that do not exist")

skills/agent-cost/SKILL.md

@@ -1,34 +0,0 @@
----
-name: agent-cost
-description: >-
-  Measure what an agent session actually costs in tokens using bin/agents/cost.
-  Use before and after changing documentation, skills or context layout, and when
-  a session feels unexpectedly expensive.
----
-
-# agent-cost
-
-```bash
-bin/agents/cost                        # every project, YAML
-bin/agents/cost --repo 2dph            # only sessions whose cwd matches
-bin/agents/cost --json | jq .cursor.by_tool
-bin/agents/cost --snapshot after-x --repo 2dph   # append a row to docs/CONTEXT-BUDGET.md
-```
-
-Reads local session storage from OpenCode and Cursor transcripts. Reports the
-always-loaded baseline, cache hit/miss/thrash, and which tools moved the most
-bytes.
-
-## How to read it
-
-- **Baseline** is what every single message pays for: `AGENTS.md` plus anything
-  eagerly linked from it. Keep it small; it multiplies by message count.
-- **Cache thrash** matters more than raw size. Editing a file that sits early in
-  the context invalidates the prompt cache for the whole session.
-- **by_tool bytes** shows where the real spend is. Usually it is unfiltered
-  command output, not documentation.
-
-## Rule
-
-Measure before and after. A claim that something "reduces tokens" without a
-before and an after number is an opinion, not a result.

skills/brain/SKILL.md

@@ -1,5 +1,5 @@
 ---
-name: kb-search
+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
@@ -8,7 +8,7 @@ description: >-
   documentation.
 ---
 
-# kb-search — deduction over facts and info
+# brain — deduction over facts and info
 
 One embedded Ladybug graph (`var/kb.lbug`, read-only when queried) holding two
 roots: