Document lock acquisition order for saveMu + subprocess hooks

[jongio]

Context

Follow-up from PR #7776 (exegraph) review — wbreza Pass 3, finding M1.

Problem

\saveMu\ in \pkg/environment/manager.go\ serializes Save/Reload. Parallel hooks invoke \�zd env set\ subprocesses which acquire the same lock chain. With many concurrent hook goroutines, inconsistent acquisition ordering is theoretically possible.

Cross-process flock + in-process mutex ordering should be documented explicitly.

Acceptance Criteria

• Add lock-acquisition order comments to \pkg/environment/manager.go\ documenting the \saveMu\ → flock ordering
• Run \go test -race\ on hook-heavy templates to validate no races exist
• Document which mutexes are held during subprocess invocations

Related

• PR feat(exegraph): graph-driven execution engine for up/provision/deploy #7776 (exegraph engine)