fix(skills): default skills install to detected agents instead of --all

[Xefreh]

Summary

Fixes #149.

firecrawl init and firecrawl setup skills always appended --all to the
underlying npx skills add call, which installs skills into every agent the
skills package (vercel-labs/skills) knows about — 60+ agents — regardless
of whether the agent is actually present on the machine. In practice this
litters $HOME with dozens of unused hidden directories (.openclaw,
.qwen, .tabnine, .kiro, .trae, .vibe, ...) for tools the user never
installed.

The codebase already has the right detection logic —
detectInstalledAgents() in src/commands/skills-native.ts — but it was
only reachable from the native no-npx fallback path, never from the default
npx-based install path that basically everyone hits.

Changes

• skills-native.ts: export detectInstalledAgentNames(), a thin wrapper
  around the existing detection.
• skills-install.ts: add resolveSkillsTarget() — explicit --agent or
  --all still win, otherwise install is scoped to agents actually detected
  on the machine, falling back to --all only if nothing is detected (so a
  non-interactive run never ends up with no target). buildSkillsInstallArgs
  no longer defaults to --all on its own, and agent can now be a list so
  multiple detected agents can be passed in one npx skills add call.
• init.ts / setup.ts: both entry points now call resolveSkillsTarget
  instead of leaving the --all default implicit.
• Updated init.test.ts / setup.test.ts to cover the new default
  (detection-based) behavior and the fallback-to---all case, isolating
  $HOME per test (mkdtempSync) so results don't depend on the machine
  running the suite.

Test plan

• npm run build passes
• npx vitest run — 363/363 tests pass
• Manually verified on a real machine: ran the built CLI's
  init -y --skip-install --skip-auth against a $HOME containing only
  ~/.claude; confirmed skills were installed into ~/.claude/skills
  and ~/.agents/skills only, with none of the other 60+ agent
  directories created.