acq: opt-in --inherit-git-identity to carry host git identity into the sandbox (env-injection) #214

Summary

Opt-in convenience (deferred from the git-identity classifier work, approved via consensus vote): let a user inherit their host global git identity into the sandbox without weakening isolation.

Background

The sandbox home is isolated — the host ~/.gitconfig is deliberately NOT mounted — so git identity only crosses the boundary via repo-local config. That's correct for isolation, but a user with many sub-repos must set user.email per repo. An opt-in inheritance mechanism would reduce that friction.

Proposed (env-injection only — the vetted approach)

Add an opt-in flag (e.g. acq run --inherit-git-identity) that reads the host's git config --global user.name / user.email and injects them as GIT_AUTHOR_NAME/GIT_AUTHOR_EMAIL + GIT_COMMITTER_NAME/GIT_COMMITTER_EMAIL into the sandbox environment.

• Only two scalar strings cross the boundary — no config-surface leak.
• Off by default (adopting host identity inside an isolation sandbox is itself a mild surprise).
• Authorship only — it does NOT configure signing; SSH signing still uses the forwarded agent key.

Explicitly rejected (consensus vote)

• Mounting ~/.gitconfig — leaks credential helpers, insteadOf URL rewrites, and signing-key paths across the boundary (security surface). Also references host key files that aren't mounted → breakage.
• Auto-writing repo-local config — mutates the user's repos and leaves state behind after the sandbox is gone (surprising, and doesn't know which of many sub-repos to touch).

Status

Deferred as YAGNI until requested — the classifier fix (this PR's sibling) already gives accurate per-repo guidance. Filing so it's tracked, not forgotten.