find: prompt package env/args inputs in search installs

[andrelandgraf]

Summary

• extend find package install planning to collect package metadata inputs (env, args, and headers metadata) and carry them into the generated install options
• keep placeholder behavior consistent with headers: required empty values are scaffolded with placeholders, optional empty values are omitted from the final config inputs
• wire args through install config building for local/package/command sources and add regression tests for find + installer flows

Test plan

• bun run tests/find.test.ts
• bun run tests/installer.test.ts
• bun run tests/e2e/cli.test.ts