Define first-run install flow for supported harnesses

[johnnyhuy]

Weldable should be the installer and launcher utility for agent harnesses such as Claude Code, Codex, OpenCode, and OpenPi.

Starter scope:

• map the first-run flow from zero to usable
• decide supported platforms and packaging path
• define how install verification should work
• capture handoff points to provider/bootstrap setup

Assignee: johnnyhuy