feat(cli): add testnode skills add to sync agent skills

[douglance]

What

Adds a testnode skills add command that generates and installs agent skill files for the CLI, so AI coding agents (Claude Code, Cursor, etc.) can discover and drive testnode.

$ testnode skills add
7 skills synced
  ~/.agents/skills/testnode-clean
  ~/.agents/skills/testnode-init
  ~/.agents/skills/testnode-logs
  ~/.agents/skills/testnode-snapshot
  ~/.agents/skills/testnode-start
  ~/.agents/skills/testnode-status
  ~/.agents/skills/testnode-stop

Why it needed wiring

incur ships a built-in skills add, but it only activates for a single root Cli. This CLI hand-rolls dispatch across two sub-Clis (start and local groups), so the built-in never ran.

How

apps/cli/src/commands/skills.ts reads each sub-Cli's command map via incur's exported Cli.toCommands, merges them, and passes the result to SyncSkills.sync("testnode", …) — the same call incur's built-in uses. One SKILL.md per command is installed to incur's default global location (~/.agents/skills/testnode-<command>/).

apps/cli/src/index.ts routes skills add and lists it in --help.

Scope

• 3 files: new skills.ts, index.ts dispatch/help, new skills.test.ts.
• No existing command behavior changes.
• pnpm test:run green (161 tests, incl. the new one); build + typecheck clean.

[douglance]

Superseded by #19, which does skills the canonical incur way (built-in skills add via the sync option) as part of refactoring the CLI to a single idiomatic incur root. Closing in favor of #19.