feat(sync): prune npx cache envs serving outdated ruflo-family code

[pacphi]

Why

npx envs (<npm-cache>/_npx/<hash>/) are snapshots keyed by requested spec: once npx @claude-flow/cli caches a version, --prefer-offline serves that copy forever — upgrading the global install never touches it. The statusline/hook npx fallbacks execute these verbatim, which is how a machine running a fixed ruflo 3.32.2 kept rendering the fabricated CVE counter from a cached 3.28.0 (#28). On the machine that surfaced this, six such envs held ~6.4 GB of retired code (ruflo 3.10 / 3.21 / 3.25, @claude-flow/cli 3.28 ×2, agentic-qe 3.11.5 — the init-clobbering era).

This automates what #28 remediated by hand: a new npx status row and a matching sync heal.

The prune rule — conservative by construction

An env is removed only when all of the following hold; a miss fails safe as "not pruned", never as a wrong prune:

• every package the env is keyed to (its package.json dependencies) is kit-managed (ruflo, @claude-flow/cli, agentic-qe) — an env we can't fully judge is exempt (so pnpm/typescript/context7/… envs are never touched)
• each has an installed global baseline to compare against — @claude-flow/cli resolves from its nested location under ruflo (the same layout fact behind the fix(statusline): resolve the real CLI bins so delegation stops falling to stale npx #28 bin fix; there is never a top-level global copy)
• at least one cached copy is strictly older than its baseline — equal-or-newer stays, since a current cache is exactly what a pre-install machine's npx fallback runs

Envs are pure caches; npx re-fetches on demand, so removal is always recoverable.

Wiring

• ak status: new npx row (warn + fix when stale, listing pkg@version)
• ak sync: prunes on the npx row or after versions upgrades — an upgrade is precisely what turns a previously-current cache stale, closing the gap in one pass
• paths.mjs: npxCacheDir() resolved from npm_config_cache or platform defaults without spawning npm; a custom userconfig cache path is missed, which only means an empty scan (documented)

Verification

End-to-end against the real cache with a synthetic stale env planted (ruflo@3.21.1, the exact shape pruned by hand in #28):

⚠ npx  1 stale npx env(s) serve outdated code (ruflo@3.21.1)  → sync prunes them
• [npx] sync prunes them (npx re-fetches on demand) — because: …   (dry-run plan)
{"ok":true,"detail":"pruned 1 env(s): ruflo@3.21.1 (npx re-fetches on demand)"}
✓ npx  npx cache holds no stale ruflo-family envs

Current and foreign envs untouched throughout.

Tests

tests/kit/npx.test.mjs — 10 hermetic tests (synthetic _npx fixtures + injected baseline; never the machine's real cache): strictly-older flagged; equal kept; newer kept; mixed managed+unmanaged exempt (partial verdicts never prune); no-baseline exempt; unreadable copy exempt; missing cache dir returns empty; prune removes exactly the stale envs; clean-cache no-op; nested @claude-flow/cli baseline resolution.

Gates: 110 mjs + 43 cjs tests pass, eslint 0, typecheck 0.