Pass host environment to Fold Bash subprocesses

[K-Mistele]

Summary

• add a host-provided processEnvironment option to Fold's Bash tool
• forward the option through codingTools so embedding hosts can configure every session subprocess
• preserve Fold's managed-bin PATH prefix while honoring a host-provided PATH
• verify direct Bash and codingTools environment propagation with real subprocess tests

Why

Riptide commit attribution installs per-session Git hook configuration through environment variables. Fold embedding hosts need a session-scoped environment seam so commits created by Bash subprocesses inherit that attribution configuration.

Verification

• bun run typecheck
• bun run test
• bun run lint
• bun run format:check