feat: interactive install wizard + pipeline support

[dutifulbob]

Summary

• Interactive mode: when --agent or --scope is omitted and a TTY is available, skill-install launches a multiselect wizard instead of erroring
• Piped stdin support: simpledoc --skill export <id> | skill-install reads the tar from stdin and opens /dev/tty for wizard prompts
• TTY arrow key fix: lazy-load @clack/prompts to prevent the producer's readline side effect from interfering with the consumer's raw mode in pipelines
• --help support: added --help/-h with a cleaner multi-section usage block
• Removed parent/admin scopes: stripped from code, CLI, wizard, docs, and tests
• Collision preflight: detect install destination collisions before writing
• Multi-agent install matrix: install to multiple agent×scope combos in one run
• README update: documented interactive mode, multi-target install, supported agents/scopes