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.

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.