P3 (kickoff): generic tools/ library + end-to-end tool-using-agent test

Stand up executus/tools — the generic, host-agnostic tool library — and prove
the full pattern end to end:

- tools/tools.go: Register(reg) adds the always-available zero-dependency tools
  (currently `think`). A light host calls it and is immediately useful; backed
  tools (web/store/meta groups) will register via grouped registrars with
  nil-safe Deps as they land.
- tools/think.go: the `think` tool moved from mort (imports only executus/tool).
- tools/integration_test.go: end-to-end proof that the executor runs an agent
  which CALLS a registered tool — the fake model emits a `think` tool call, the
  executor dispatches it through the registry, the model finalises, and the step
  instrumentation captures the `think` step. Exercises the full tool-dispatch
  loop through run.Executor.

Stacked on phase-2-run-kernel (P3 needs run.Executor). Remaining P3: the
meta/web/net/store/compose groups + their Deps + default backends (splitting
mort's default.go grab-bag).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -58,7 +58,11 @@ CORE (majordomo + stdlib):
              structured output — no separate structured/ pkg)
   llmmeta/  shared meta-LLM helper over model/            [P1 ✓]
   compact/  context compactor (WithCompactor hook)       [P2 ✓]
-  tools/{web,net,store,compose,meta,comms}  generic tools [P3]
+  tools/    generic tool library + Register entrypoint;  [P3 wip]
+            think moved; end-to-end "agent calls a tool"
+            test green. Remaining: meta/web/net/store/
+            compose groups + their nil-safe Deps + default
+            backends (the default.go grab-bag split)      [P3]
 
 BATTERIES (opt-in siblings, each nil-safe + a default):
   persona/   Agent noun + AgentStore seam + yml loader   [P4]