Add the 'early-return' linter, pass more ctx

[djeebus]

Also tweak some integration tests to try and make them less flaky. Success?

---

[!NOTE] Enable the early-return linter and refactor code and tests to use early returns while stabilizing integration tests with EventuallyWithT and longer timeouts.

• Linting:
  • Enable revive rule early-return in .golangci.yml.
• Client Proxy (packages/client-proxy/main.go):
  • Simplify Redis and secure-Redis initialization with early-return on errors; warn and fallback to in-memory catalog when Redis is disabled.
• Orchestrator NBD (packages/orchestrator/internal/sandbox/nbd/dispatch.go):
  • Refactor request parsing loop to early-break on incomplete headers and flatten command handling.
  • Gate cmdRead/cmdWrite with early shutdown checks and move pendingResponses.Add(1) accordingly.
• Template Build (.../template/build/phases/steps/builder.go):
  • Early-return when layer is cached; otherwise log and proceed to build.
• Storage Lock (packages/shared/pkg/storage/lock/file_lock.go):
  • Streamline TTL check: return ErrLockAlreadyHeld when fresh; otherwise clean up stale lock before acquiring.
• Integration Tests:
  • Replace require.Eventually with require.EventuallyWithT, increase timeouts/poll intervals, reuse ctx, and add retrying teardown to reduce flakiness.

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[jakubno]

Feel free to cancel my review, I can't do it when there's merge conflict