Investigate environment lock contention under parallel deploys

[jongio]

Context

Follow-up from PR #7776 (exegraph) review - wbreza Pass 3, finding M4.

Problem

Every Getenv/LookupEnv call in pkg/environment/environment.go acquires mu.RLock individually. Under 30+ concurrent goroutines (e.g. parallel service deploys all reading env vars), this may become measurable contention.

Acceptance Criteria

• Profile env read contention under realistic parallel workloads (30+ goroutines)
• If measurable: consider batching reads, sync.Map, or snapshot-on-write pattern
• If not measurable: close as won't-fix with profiling evidence

Notes

Not catastrophic - current impl is correct. This is a performance investigation, not a correctness fix.

Related

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