acq: opt-in --create flag to make a missing workspace directory #215

Summary

Optional convenience (deferred from the workspace-path pre-flight work, noted in consensus vote): an opt-in --create flag so acq run/acq create can create a missing workspace directory instead of failing.

Background

As of the pre-flight change, acq run <agent> <missing-path> fails fast with an actionable mkdir -p <path> && acq run … hint (rather than passing a bad path to sbx create). This is the safe default: it never silently creates host directories (a typo like ~/gti/brokers shouldn't materialize a wrong dir), and it's CI/non-interactive-safe.

For the deliberate "new workspace" flow, a user currently runs one extra mkdir -p. An explicit opt-in flag would remove that step for users who want it.

Proposed

acq run --create <agent> <path> (and acq create --create …): if <path> does not exist, create it (mkdir -p) with a printed notice, then proceed. Without the flag, behavior is unchanged (fail fast + hint).

• Off by default — explicit opt-in only, so no surprising host-filesystem mutation.
• Non-interactive-safe (a flag, not a prompt) — works in CI.
• Still fails on a path that exists but is a file (not a directory).

Status

Deferred as YAGNI until requested — the one-line mkdir -p hint covers the flow today. Filing so it's tracked. Related: the pre-flight change in the workspace-path-guidance PR; sibling of the --inherit-git-identity deferral.