Amp Runners Add Git Worktree Creation and Secret Injection

Amp runners can now create Git worktrees from the directory picker, with each new worktree placed as a sibling folder on a new branch off the current HEAD while uncommitted changes stay in the original checkout.

Runners can also opt in with --amp-env to inject Secrets & Env Vars configured on ampcode.com into thread shell commands, MCP servers, and plugins, with changes applied to the next thread without a restart.