feat: the versioned tool cache #339

Why

No global installs (design: Tools without global installs).

In scope

Install to $XDG_CACHE_HOME/agentic-kit/tools/@ with npm install --prefix --allow-scripts=, build and check natives there, rename into place atomically; prune to pinned versions plus 30 days.

Out of scope

Launchers (#340).

Acceptance criteria

• Two projects on different Ruflo versions do not interfere
• An interrupted install leaves no partial folder
• Pruning keeps every pinned version

Dependencies

Blocked by: #330

Blocks: #340, #341, #343, #347, #351, #372, #385

Plan

• Release: 4.0.0-beta.1
• Size: size:M · Workstream: ws:project-scope
• Epic: feat: project-scope P2, tool cache and launchers (epic) #297
• Card: P2-01 in the card inventory
• Sources: Design (docs(plans): project-scoped management design and its prerequisites #289) Tools without global installs

Verification

• node scripts/run-tests.mjs focus (new cache tests)