Investigate environment lock contention under parallel deploys

## 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 #7776 (exegraph engine)